gemfile: Update `gitaly` to `16.1.0-rc2`

Update the gitaly gem from `16.1.0-rc1` to `16.1.0-rc2`. The new gem has
the updated protobuf for `GetTreeEntries` RPC, which includes new
structured error type `GetTreeEntriesError`.

In the following commit[s], we will use this error for better response
handling.

The new release of the gitaly gem also deprecates the
`Gitaly::FindLocalBranchResponse` field. So modify `ref_service_spec.rb`
to remove any usage of the field.