Update Gitaly to v15
This commit also excludes gitlab-shell from dependencies:
Gitaly specifies Gitlab Shell as a dependency as well in order to use gitlabnet client to perform API endpoints to Gitlab Rails. As a result, Gitlab Shell requires Gitaly -> Gitaly requires an older version of Gitlab Shell -> that version requires an older version of Gitlab Shell, etc. Let's use exclude to break the chain earlier
Related issue: https://gitlab.com/gitlab-org/gitlab/-/issues/368953
Edited by Igor Drozdov