Use transaction on model instead of connection

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/336900

lib/gitlab/optimistic_locking.rb

@@ -11,7 +11,7 @@ module Gitlab
       retry_attempts = 0
 
       begin
-        ActiveRecord::Base.transaction do # rubocop: disable Database/MultipleDatabases
+        subject.transaction do
           yield(subject)
         end
       rescue ActiveRecord::StaleObjectError