Fix users autocomplete in a subgroup

What does this MR do?

When doing user autocomplete in a subgroup, look for users who have access from parent groups too.

Are there points in the code the reviewer needs to double check?

Don't think so, although it doesn't look like Group#users_with_parents was used at all in CE before I added this 🤔

Why was this MR needed?

This broke group issue boards in subgroups. That's an EE feature, but the fix belongs in CE too.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/39452.