Upgrade GitLab Workhorse to v8.50.0

This brings in
https://gitlab.com/gitlab-org/gitlab-workhorse/-/merge_requests/619,
which updates Gitaly's latest Protocol Buffer definitions. This is
needed to support a new flag (`IncludeLfsBlobs`) in the
`GetArchiveRequest` RPC for
https://gitlab.com/gitlab-org/gitlab/-/issues/15079.

GITLAB_WORKHORSE_VERSION

-8.49.0
+8.50.0