Expose cleanup policies limits and document them

🐦 Context

Users using the container registry accumulate image tags. Those tags take up physical space on the Container Registry backend.

To keep the space usage in check, we introduced cleanup policies in 12.8.

Those policies are executed by background workers. Given the amount tags they might need to delete, we implemented throttling on these workers. See !40740 (merged) and !49961 (merged).

Those limits have been added behind a feature flag while we tweak them for gitlab.com . Here is the rollout issue: #238190 (closed).

Having said that, self-managed users might be interested to use those limits.

🔬 What does this MR do?

• Expose all the available cleanup policies limits in the related Application Settings panel if the feature flag is enabled.
• Document all the limits and how to enable them for self-managed users.

📷 Screenshots (strongly suggested)

From /admin/application_settings/ci_cd:

Does this MR meet the acceptance criteria?

📏 Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team