Update Legacy Rake Restore Task to Ignore New Postgresql Error

Support raised an issue to our attention: https://gitlab.com/gitlab-com/request-for-help/-/issues/2763

As examined in gitlab-com/gl-infra/data-access/durability/team#120 (closed), PG16 inadvertently introduces a new error message that is harmless on single-node installations: must be able to SET ROLE "gitlab-psql".

This issue is to update IGNORED_ERRORS in lib/backup/targets/database.rb to disregard this new error going forward.