Fix rename_login_root_namespaces post migration

changelogs/unreleased/51782-fix_rename_login_namespace_migration.yml

+---
+title: Fix migration to avoid an exception during upgrade
+merge_request: 22055
+author:
+type: fixed

db/post_migrate/20180816193530_rename_login_root_namespaces.rb

@@ -5,6 +5,8 @@ class RenameLoginRootNamespaces < ActiveRecord::Migration
 
   DOWNTIME = false
 
+  disable_ddl_transaction!
+
   # We're taking over the /login namespace as part of a fix for the Jira integration
   def up
     disable_statement_timeout do