Check group member permissions in GroupMentionService

What does this MR do and why?

Fixes https://gitlab.com/gitlab-org/gitlab/-/issues/425067 by checking group member access to the context of a group mention, before sending a group mention notification.

How to set up and validate locally

1. Enabled the :group_mention_access_check feature flag.
2. Create a Slack Incoming Webhook to receive notifications (https://github.com/darklynx/request-baskets works great).
3. Create a Public group called slack-notifications-test-group and configure Slack notifications with the webhook and both A group is mentioned... triggers enabled.
4. Add a non-admin user to slack-notifications-test-group with Reporter permissions.
5. Create a Public project (outside of slack-notifications-test-group).
6. Mention @slack-notifications-test-group in a public issue in the project. A Slack notification should arrive.
7. Mention @slack-notifications-test-group in a confidential issue in the project. No Slack notification should arrive.
8. Remove the non-admin user with the Reporter permissions from @slack-notifications-test-group.
9. Mention @slack-notifications-test-group in a confidential issue in the project. A Slack notification should arrive.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.