Migrate user records to ghost in batch job runs

What does this MR do and why?

It splits Users::DestroyService into a 2 steps workflow:

• keeps the first part of Users::DestroyService
• migrating associated records to the ghost user and deleting records is migrated to Users::MigrateRecordsToGhostUserService

Migration and deletion is now done in batches asynchronously with a limited execution time:

1. Users::MigrateRecordsToGhostUserInBatchesWorker is a cron worker running every minutes
2. The worker executes Users::MigrateRecordsToGhostUserInBatchesService with execution time limit of 30 seconds
3. Users::MigrateRecordsToGhostUserService is executed for each user

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/366655.

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app/models/users/migrate_to_ghost_user.rb

+# frozen_string_literal: true
+
+module Users
+  class MigrateToGhostUser < ApplicationRecord
+    self.table_name = 'migrate_to_ghost_user'
+
+    belongs_to :user
+    belongs_to :initiator_author, class_name: 'User'
+
+    validates :user_id, presence: true
+  end
+end