Transition to scoped feature/tooling labels

This is a preparatory step before we deprecate and move away from the
`backstage` label.

Next step will be removing references to `backstage`:
https://gitlab.com/gitlab-org/gitlab/-/issues/219751

See
https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/488#proposal


for the overall plan.

Signed-off-by: Rémy Coutable <remy@rymai.me>