Add user namespace to the free user cap alert

What does this MR do and why?

Adds an alert on personal project overview and member pages for the free user cap - see #360018

this preview alert is behind the free_user_cap feature flag, so there is no changelog.

Related to #360018

Screenshots or screen recordings

How to set up and validate locally

1. Simulate SaaS and restart GDK
2. Update Application Setting

   bin/rails c
   ApplicationSetting.first.update(check_namespace_plan: true)

3. Enable the feature flag

   Feature.enable(:free_user_cap)

4. Disable preview free user cap feature flag

   Feature.disable(:preview_free_user_cap)

5. Visit any personal project http://127.0.0.1:3000/root/test-admin-namespace-project
6. Add at least 5 members.
7. Refresh the project members page you are on or go to the project overview page to view the alert.

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.