Ensure BacthedMigration and BatchedJob always have pause_ms attribute

Define `pause_ms` attribute for both `BatchedMigration` and `BatchedJob`
models to make sure older migrations will execute even if the column is
not created in the DB yet.

See https://gitlab.com/gitlab-org/gitlab/-/issues/328816.

lib/gitlab/database/background_migration/batched_job.rb

@@ -17,6 +17,8 @@ module Gitlab
 
         delegate :aborted?, :job_class, :table_name, :column_name, :job_arguments,
           to: :batched_migration, prefix: :migration
+
+        attribute :pause_ms, :integer, default: 100
       end
     end
   end

lib/gitlab/database/background_migration/batched_migration.rb

@@ -23,6 +23,8 @@ module Gitlab
           finished: 3
         }
 
+        attribute :pause_ms, :integer, default: 100
+
         def self.active_migration
           active.queue_order.first
         end