Fix an unsafe type assertion in google_artifact_registry_repository

[modular-magician]

Fixes #21900 which is really a dupe of #21348. The latter also mentions a perma-lack-of-diff which this PR doesn't address (but the pending PR for the latter does). This PR only gets past the panic, but the lack of diff should be able to be worked around by changing the id of the policy a la:

   cleanup_policies {
     id     = "delete_older"
     action = "DELETE"
     condition {
       older_than = "62d"
     }
   }

to:

   cleanup_policies {
     id     = "delete_much_older"
     action = "DELETE"
     condition {
       older_than = "162d"
     }
   }

artifactregistry: fixed type assertion panic `google_artifact_registry_repository`

Derived from GoogleCloudPlatform/magic-modules#13445