Geo: Rename replication_enabled_replicator_classes for clarity

The following discussion from !157697 (merged) should be addressed:

• @dbalexandre started a discussion:

  note: For a later MR:

      def self.replication_enabled_replicator_classes

Implementation Guide

• Rename the method Gitlab::Geo.enabled_replicator_classes to Gitlab::Geo.replication_enabled_replicator_classes
• Search for calls to enabled_replicator_classes in gitlab codebase
• Identify which ones are calls on Gitlab::Geo and which ones are calls on ee/app/helpers/ee/geo_helper.rb's enabled_replicator_classes.
• Replace the calls to Gitlab::Geo.enabled_replicator_classes with Gitlab::Geo.replication_enabled_replicator_classes