Currently, `gitlab_rails` service needs to be enabled in order for
`gitaly` service to function properly. Specifically, `gitlab_rails`
creates `git_data_dirs` (e.g. `/var/opt/gitlab/git-data/repositories`)
which is needed for Gitaly to start-up. If only the `gitaly` service is
enabled, then Gitaly won't start due to config validation error (i.e.
storage directories have to be present).

This MR makes the creation of these directories the responsibility of
the `gitaly` cookbook. The `gitlab` cookbook still creates the
directories as backward-compatibility measure, as I'm not sure if these
directories are needed outside of Gitaly.

This is needed as part of
https://gitlab.com/gitlab-com/gl-infra/production-engineering/-/issues/24530.

Changelog: fixed