Allow administrators to set active pipelines limits

What does this MR do and why?

Allows admins to change the ci_active_pipelines limit.

Screenshots or screen recordings

How to set up and validate locally

On GDK:

1. Go to Admin Area
2. Click Settings > CI/CD in the sidebar menu
3. Expand Continuous Integration and Deployment
4. Edit some of the limits for different plans
5. Click Save <plan> limits button
6. Ensure all the limits are changed correctly

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.