Clean up the post deployment migration generator (#24233)

lib/generators/post_deployment_migration/USAGE

@@ -0,0 +1,10 @@
+Description:
+    Generate a Rails migration in the db/post_migrate/ dir.
+
+    Interacts with the post_deployment_migrations initializer.
+
+Example:
+    bin/rails generate post_deployment_migration IsolateChanges
+
+    Creates a migration in db/post_migrate/<timestamp>_isolate_changes.rb
+    which will have `disable_ddl_transaction!` and a `change` method included.