Expose membership_lock on group detail API

What does this MR do and why?

This MR exposes the membership_lock field so this can be implemented as part of the GitLab Terraform provider.

How to set up and validate locally

1. Run in EE
2. Update a group via an API call to set membership_locked to true
3. Validate that membership_locked returns in the API response and any GET responses for group details.

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.