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.

lib/generators/post_deployment_migration/post_deployment_migration_generator.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+require 'rails/generators/active_record'
+
+class PostDeploymentMigrationGenerator < Rails::Generators::NamedBase
+  source_root File.expand_path('templates', __dir__)
+
+  include Rails::Generators::Migration
+
+  def create_post_deployment_migration
+    migration_template 'migration.erb', "db/post_migrate/#{file_name}.rb"
+  end
+
+  def self.next_migration_number(path)
+    ActiveRecord::Generators::Base.next_migration_number(path)
+  end
+end

lib/generators/post_deployment_migration/templates/migration.erb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class <%= migration_class_name %> < ActiveRecord::Migration[<%= ActiveRecord::Migration.current_version %>]
+  disable_ddl_transaction!
+
+  def change
+  end
+end