This change set implements a new `/metadata` REST API endpoint by
leveraging the already existing GraphQL `Query.metadata`.
Therefore, this endpoint is inherently *on par* with the GraphQL API.

This new endpoint somewhat makes the already existing `/version`
REST API endpoint obsolete. The reason for not just extending the
`/version` endpoint with the missing metadata is because the GraphQL API
also uses the *metadata* wording for that endpoint. I think the more
consistency the better here.

A follow up may (???) therefore properly deprecate the
`/version` REST API endpoint in favor of the new `/metadata` endpoint.

Changelog: added
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/87622
Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/357032
Epic: https://gitlab.com/groups/gitlab-org/-/epics/7204