Pass user instance to Labels::FindOrCreateService or skip_authorization: true

It fixes a bug described in #23694 when `project.owner` was passed to `Labels::FindOrCreateService`. `Labels::FindOrCreateService` expected a user instance and `project.owner` may return a group as well. This MR makes sure that we either pass a user instance or `skip_authorization: true`.

Fixes #23694

See merge request !7093
Signed-off-by: Rémy Coutable <remy@rymai.me>