convert declarative pipeline to scripted pipeline
The scripted equivalent to the declarative
steps {
  // pipeline steps here
post {
  failure {
    // failure handler here
  }
  success {
    // success handler here
  }
  always {
    // always handler here
  }
}

is the native Groovy try { } catch { }:
try {
  // pipeline steps here

  // success handler here
} catch(Exception e) {
  // failure handler here
} finally {
  // always handler here
}

I'm not sure off the top of my head how to add an abort handler, but I think you might be able to do it by inspecting the currentBuild.result variable in the failure handler block.