[repo] move repo.gc_stats to Service API (closes #2951068) We currently have a method on the repo that anyone can call. `call_service` gives a similar thing, modulo: * can be restricted access to manager only (thanks to selection on services), * less unrelated code on the repository class, * no need to fetch a reference to a repo object from the client side * (`req._cnx.repo` is not an API and config.repository() is on its way out). The old way to access this information (repo.gc_stats()) is deprecated.

[repo] move repo stats to Service (closes #2951067) We currently add a new method on repo that anyone can call. Call service allows to reach the same result with: * Restricted access to manager only (thanks to selection on services), * Less unrelated code on the repo. * No need to fetch a reference to a repo object from client side. `req._cnx.repo` is not an API and config.repository() is on its way out. The old way to access this information (repo.stats()) is deprecated.