Add dashboard limit to enable free user management

What does this MR do and why?

• adds dashboard limit application settings for the free user cap rollout
• adds UI for the new settings to only be used on .com

more details thread - https://gitlab.com/gitlab-org/gitlab/-/issues/371201#note_1103389486 and main issue https://gitlab.com/gitlab-org/gitlab/-/issues/371201

Screenshots or screen recordings

How to set up and validate locally

1. Setup your GDK to simulate saas and restart.
2. run rails migrations from command line bundle exec rails db:migrate
3. Go to admin -> Settings -> General or url like this http://gdk.test:3000/admin/application_settings/general
4. Observe/Modify/etc Free User Cap section.

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 #371201