Deprecate git_data_dirs setting

[Normo]

Starting in GitLab 18.0, git_data_dirs will no longer be a supported means of configuring Gitaly storage locations.
See https://docs.gitlab.com/omnibus/settings/configuration.html#migrating-from-git_data_dirs

Relevant lines:

• https://github.com/hifis-net/ansible-collection-toolkit/blob/v5.3.0/roles/gitlab/templates/gitlab.rb.j2#L34
• https://github.com/hifis-net/ansible-collection-toolkit/blob/v5.3.0/roles/gitlab/templates/gitlab.rb.j2#L39