title: error CS8773: "Feature 'global using directive' is not available in C# 9.0" after downgrade from net6.0 to net5.0
body: <p>I have a project that was initially created for .NET 6 but then I needed to downgrade it to .NET 5.
  I changed Target framework in Project Properties and tried to compile. As a result I received a bunch of the errors:</p>
  <pre><code>GlobalUsings.g.cs(2,1,2,29): error CS8773: Feature 'global using directive' is not available in C# 9.0. Please use language version 10.0 or greater.
  </code></pre>
  <p>File GlobalUsings.g.cs is created automatically and it reappears every time after compilation.</p>