Use fair queueing for Loose Foreign Keys

This change enables the LFK fair queueing mechanism to avoid "stuck" or
delayed processing when objects with large number of child records are
cleaned up.

Changelog: added

app/services/loose_foreign_keys/batch_cleaner_service.rb

@@ -54,7 +54,7 @@ module LooseForeignKeys
     attr_reader :parent_table, :loose_foreign_key_definitions, :deleted_parent_records, :modification_tracker, :deleted_records_counter, :deleted_records_rescheduled_count, :deleted_records_incremented_count
 
     def handle_over_limit
-      return if Feature.disabled?(:lfk_fair_queueing)
+      return if Feature.disabled?(:lfk_fair_queueing, default_enabled: :yaml)
 
       records_to_reschedule = []
       records_to_increment = []

config/feature_flags/development/lfk_fair_queueing.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/351082
 milestone: '14.8'
 type: development
 group: group::sharding
-default_enabled: false
+default_enabled: true