Draft: Feat(overage_members_modal): remove feature flag

What does this MR do and why?

Remove the overage_members_modal feature flag after having been rolled out in production since January 2023.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Invite modal	Invited

How to set up and validate locally

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

1. Visit any group page, then Group Information -> Members: http://localhost:3000/groups//-/group_members.
2. Click the Invite members button
3. Add a user and click Invite.
4. Check the modal.

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.

Related to #388124