Handle storage scoped mutators in per repository router

Gitaly's namespace operations contain some storage scoped mutator RPCs. There appears to still be some invocations of those RPCs. We should look in to them and see whether they can be rescoped or whether we should handle them in the per repository router.

Dashboard: https://dashboards.gitlab.net/d/000000199/gitaly-feature-status?orgId=1&from=now-6h&to=now&refresh=30s&var-environment=gprd&var-method=RenameNamespace&var-prometheus=prometheus-01-inf-gprd

The following discussion from !2731 (merged) should be addressed:

• @pks-t started a discussion: (+1 comment)

  It is default now, but don't we need to cope with non-hashed storages still? As long as they're supported, I don't think we can just ignore them. I was under the impression that e.g. on GitLab.com we're still using both types.