Custom MR approval rule does not work with subgroups
# Summary
Creating a new approval rule specifying only the parent group does not allow members of subgroups to approve, regardless of their role.
Split from: https://gitlab.com/gitlab-org/gitlab/-/issues/389374#note_1329358902 as per @abdwdd request.
# Steps to reproduce
Create the following setup:
* Create `group/subgroup` containing user Alice with Developer role
* Inside `group` members, invite `subgroup` with Developer role as max level (note that Alice is part of `group` only through `subgroup`)
* Create `project` inside `group`
* In the `project` settings, create a custom approval rule with `group` (not `subgroup`!) constituting the approval list
# Example Project
[This shows the setup](https://gitlab.com/example-project-for-issue-filtering-bug/project/-/issues).
However note that Gitlab Premium is needed for Merge Request Approval rules, and this was created on a free account.
## What is the current *bug* behavior?
The user part of the `subgroup` will not be able to approve the MR unless he is a direct member of `group` too.
## What is the expected *correct* behavior?
The user part of the `subgroup` will be able to approve the MR via the custom approval rule.
## Relevant logs and/or screenshots

## Workaround
Add subgroup explicitly to Approval Rule
issue