Update labels from backstage to feature::maintenance

Description

Update the labels applied with Renovate MRs like gitlab-org/gitlab!36689 (merged) which I believe are re-creating the ~"backstage" label on the project level. https://gitlab.com/gitlab-org/gitlab/-/labels/15614818/edit is an example of the current project level ~"backstage"

Issues: https://gitlab.com/gitlab-org/gitlab/-/issues?label_name%5B%5D=backstage Merge Requests: https://gitlab.com/gitlab-org/gitlab/-/merge_requests?label_name%5B%5D=backstage

The first item created was gitlab-org/gitlab!36689 (merged) which I believe had the label created when the MR was opened.

backstage [DEPRECATED] has been deprecated as apart of https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/488