Ensure the presence of required attrs in BBM dict

What does this MR do and why?

We need to make few attrs mandatory to support BBM dependencies framework. Eg: Run a cron to create a new issue after 'finaliz... (#424886) requires introduced_by_url, finalize_after values to be present.

Highlights:

• Renamed existing BackgroundMigration/MissingDictionaryFile cop to BackgroundMigration/DictionaryFile, to support missing keys offense too.
• Added the grace period to avoid any failures in the default branch pipelines.
• Included enforcedSince to not include existing migrations without required keys.
  • Created #428935 (closed) to handle those.

How to set up and validate locally

1. Enqueue a new batched background migration (my_batched_migration).

   bundle exec rails g batched_background_migration my_batched_migration --table_name=projects --column_name=id --feature_category=database

2. With default values in the dictionary (empty introduced_by_url and finalize_after keys), it should throw an offense

   bundle exec rubocop --cache false db/post_migrate/20231020150948_queue_my_batched_migration.rb
   
   db/post_migrate/20231020150948_queue_my_batched_migration.rb:8:1: C: BackgroundMigration/DictionaryFile: Mandatory key 'finalize_after' is missing from the dictionary. Please add with an appropriate value.
   class QueueMyBatchedMigration < Gitlab::Database::Migration[2.1] ...
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

3. Add a value for finalize_after in db/docs/batched_background_migrations/my_batched_migration.yml, should still throw an offense for missing introduced_by_url.

   bundle exec rubocop --cache false db/post_migrate/20231020150948_queue_my_batched_migration.rb
   Inspecting 1 file
   C
   
   Offenses:
   
   db/post_migrate/20231020150948_queue_my_batched_migration.rb:8:1: C: BackgroundMigration/DictionaryFile: Mandatory key 'introduced_by_url' is missing from the dictionary. Please add with an appropriate value.
   class QueueMyBatchedMigration < Gitlab::Database::Migration[2.1] ...
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   
   1 file inspected, 1 offense detected

4. Add a value for introduced_by_url too, now it shouldn't throw any offense.

   bundle exec rubocop --cache false db/post_migrate/20231020150948_queue_my_batched_migration.rb
   Inspecting 1 file
   .
   
   1 file inspected, no offenses detected

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #424883 (closed)