Allow Groups to update remove_dormant_members

What does this MR do and why?

This adds the ability for Top-level Groups to Update remove_dormant_members setting (enable/disable) and remove_dormant_members_period, implementing #461339 (closed) and #461335 (closed).

This is behind a Feature Flag group_remove_dormant_members.

MR Cadence

Description	MR
Add remove_dormant_members_period setting to namespace_settings	!153099 (merged)
Allow Groups to update remove_dormant_members_period	👈 you're here

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.

Screenshots or screen recordings

Screenshot	Screen recording
	fe_remove_dormant_members

How to set up and validate locally

1. Enable group_remove_dormant_members feature flag
2. Navigate to a Top-level group
3. Go to Settings > General
4. Expand Permisisions and group features section
5. Find the Remove dormant users field
6. Update the value
7. Click on Save Changes