Update Omnibus global server hook guidance based on v15 changes

What does this MR do?

This MR aims to provide updated details on how to configure global server hooks, aligning with the changes & deprecation that landed in GitLab Omnibus v15:

• Move custom_hooks_dir setting from GitLab Shell to Gitaly

As of GitLab Omnibus v15 it appears that:

1. A default global server hook directory is no longer enabled, and must be explicitly set by uncommenting gitaly['custom_hooks_dir'] within the gitlab.rb configuration. The pre-filled directory path on this value is set to "/var/opt/gitlab/gitaly/custom_hooks"

   • https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/15-0-stable/files/gitlab-cookbooks/gitaly/attributes/default.rb#L52
   • https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/15-0-stable/files/gitlab-config-template/gitlab.rb.template#L2356

2. It's no longer possible to setup single server hooks globally, for example with a pre-receive hook located at /var/opt/gitlab/gitaly/custom_hooks/pre-receive. Instead it looks like it's required that global server hooks are setup under a subdirectory, for example with a pre-receive hook located at /var/opt/gitlab/gitaly/custom_hooks/pre-receive.d/*. In this case it looks like the current guidance on "To create many server hooks" has to be applied irrespective of if only a single server hook is being configured.

   • It looks like for global server hooks, Gitaly only processes against <custom_hooks_dir>/hooks/<hook_name>.d/* which would explain this behavior.
     • https://gitlab.com/gitlab-org/gitaly/-/blob/master/internal/gitaly/hook/custom.go#L29

Related issues

This was discovered during troubleshooting and testing for a customer ticket:

• Zendesk - internal use only

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