Include subgroups in mentions autocomplete

What does this MR do and why?

When querying groups the user has access to, we include groups where the user has inherited membership from ancestor groups

Query changes

• User#authorized_groups

  Old query: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/28812/commands/89915

  New query: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/28812/commands/89917

• GroupsFinder#execute (from /dashboard/groups)

  Old query: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/28812/commands/89921

  New query: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/28812/commands/89923

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Create a group and a subgroup. Make sure the subgroup does not have any child projects
2. Add a user as a member of the group
3. On the group's epic description field, type @

Verify that the subgroup shows up in the autocomplete

Related to #215070 (closed)