Support multiple databases for batched background migrations

- Change BatchedJobTransitionLog to use a SharedModel

Changelog: added

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/351585

lib/gitlab/database/background_migration/batched_job_transition_log.rb

@@ -3,7 +3,7 @@
 module Gitlab
   module Database
     module BackgroundMigration
-      class BatchedJobTransitionLog < ApplicationRecord
+      class BatchedJobTransitionLog < SharedModel
         include PartitionedTable
 
         self.table_name = :batched_background_migration_job_transition_logs

spec/lib/gitlab/database/background_migration/batched_job_transition_log_spec.rb

@@ -3,6 +3,8 @@
 require 'spec_helper'
 
 RSpec.describe Gitlab::Database::BackgroundMigration::BatchedJobTransitionLog, type: :model do
+  it { is_expected.to be_a Gitlab::Database::SharedModel }
+
   describe 'associations' do
     it { is_expected.to belong_to(:batched_job).with_foreign_key(:batched_background_migration_job_id) }
   end