title: openapi-generator-maven-plugin upgrade openapi spec to 3.1.0
body: <p>I am using plugin openapi-generator-maven-plugin in version 5.3.1.</p>
  <p>Upgrading openapi spec from 3.0.3 to 3.1.0 causes generation phase errors:</p>
  <p>Spec sample:</p>
  <pre><code>{
    &quot;openapi&quot;: &quot;3.1.0&quot;,
    &quot;info&quot;: {
      &quot;title&quot;: &quot;Spec sample&quot;,
      &quot;version&quot;: &quot;1.0.0&quot;
    },
    &quot;paths&quot;: {
      &quot;/users&quot;: {
        &quot;get&quot;: {
          &quot;responses&quot;: {
            &quot;200&quot;: {}
          }
        }
      }
    }
  }
  </code></pre>
  <p>plugin error:</p>
  <pre><code>[INFO] Output directory ((removed)\target\generated-sources\openapi) does not exist, or is inaccessible. No file (.openapi-generator-igno
  re) will be evaluated.
  [WARNING] (removed).json [0:0]: unexpected error in Open-API generation
  java.lang.RuntimeException: Issues with the OpenAPI input. Possible causes: invalid/missing spec, malformed JSON/YAML files, etc.
      at org.openapitools.codegen.DefaultGenerator.generate (DefaultGenerator.java:865)
      at org.openapitools.codegen.plugin.CodeGenMojo.execute (CodeGenMojo.java:754)
      at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:137)
      at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:210)
      at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:156)
      at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:148)
      at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:117)
      at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:81)
      at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build (SingleThreadedBuilder.java:56)
      at org.apache.maven.lifecycle.internal.LifecycleStarter.execute (LifecycleStarter.java:128)
      at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:305)
      at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:192)
      at org.apache.maven.DefaultMaven.execute (DefaultMaven.java:105)
      at org.apache.maven.cli.MavenCli.execute (MavenCli.java:957)
      at org.apache.maven.cli.MavenCli.doMain (MavenCli.java:289)
      at org.apache.maven.cli.MavenCli.main (MavenCli.java:193)
      at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0 (Native Method)
      at jdk.internal.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.java:62)
      at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke (DelegatingMethodAccessorImpl.java:43)
      at java.lang.reflect.Method.invoke (Method.java:566)
      at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced (Launcher.java:282)
      at org.codehaus.plexus.classworlds.launcher.Launcher.launch (Launcher.java:225)
      at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode (Launcher.java:406)
      at org.codehaus.plexus.classworlds.launcher.Launcher.main (Launcher.java:347)
  [ERROR]
  java.lang.RuntimeException: Issues with the OpenAPI input. Possible causes: invalid/missing spec, malformed JSON/YAML files, etc.
      at org.openapitools.codegen.DefaultGenerator.generate (DefaultGenerator.java:865)
      at org.openapitools.codegen.plugin.CodeGenMojo.execute (CodeGenMojo.java:754)
      at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:137)
      at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:210)
      at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:156)
      at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:148)
      at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:117)
      at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:81)
      at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build (SingleThreadedBuilder.java:56)
      at org.apache.maven.lifecycle.internal.LifecycleStarter.execute (LifecycleStarter.java:128)
      at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:305)
      at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:192)
      at org.apache.maven.DefaultMaven.execute (DefaultMaven.java:105)
      at org.apache.maven.cli.MavenCli.execute (MavenCli.java:957)
      at org.apache.maven.cli.MavenCli.doMain (MavenCli.java:289)
      at org.apache.maven.cli.MavenCli.main (MavenCli.java:193)
      at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0 (Native Method)
      at jdk.internal.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.java:62)
      at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke (DelegatingMethodAccessorImpl.java:43)
      at java.lang.reflect.Method.invoke (Method.java:566)
      at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced (Launcher.java:282)
      at org.codehaus.plexus.classworlds.launcher.Launcher.launch (Launcher.java:225)
      at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode (Launcher.java:406)
      at org.codehaus.plexus.classworlds.launcher.Launcher.main (Launcher.java:347)
  
  </code></pre>
  <p>Previous version of openapi works fine..</p>
  <pre><code>{
    &quot;openapi&quot;: &quot;3.0.3&quot;,
    &quot;info&quot;: {
      &quot;title&quot;: &quot;Spec sample&quot;,
      &quot;version&quot;: &quot;1.0.0&quot;
    },
    &quot;paths&quot;: {
      &quot;/users&quot;: {
        &quot;get&quot;: {
          &quot;responses&quot;: {
            &quot;200&quot;: {}
          }
        }
      }
    }
  }
  
  </code></pre>
  <p>I couldn't find information if plugin supports that version of spec, but error message is very vauge.</p>
  <p>How to use openapi-generator-maven-plugin with spec 3.1.0?</p>