Support `shared_runners_setting` for Gitlab Group

Closes #6099 (closed).

This MR adds support for shared_runners_setting attribute for Gitlab Group resource.

To clarify, shared_runners_enabled is not an attribute for Group, but for Project.

Requires:

• ✅ https://github.com/xanzy/go-gitlab/pull/1805

Ref:

• API doc: https://docs.gitlab.com/ee/api/groups.html
• Options for shared_runners_setting