Validate storage limits

What does this MR do and why?

This MR addresses: https://gitlab.com/gitlab-org/gitlab/-/issues/416114

This MR enhances the PlanLimits by adding validations to attributes notification_limit, enforcement_limit, and storage_size_limit attributes.

NOTE: Validating the model might be risky and complicated in this case.

We can even skip the validation if needed, but the controller validation is still beneficial. more details: https://gitlab.com/gitlab-org/gitlab/-/issues/416114#note_1487278682

Changes made in this MR:

• Added custom validations to ensure that notification_limit is greater than or equal to storage_size_limit.
• Added custom validations to ensure that enforcement_limit is greater than or equal to storage_size_limit.
• Added custom validations to ensure that enforcement_limit is greater than or equal to notification_limit.
• Updated the test cases to cover the new validations for all three attributes.

Validation steps:

• Go to the admin page at http://gdk.test:3000/admin/namespace_limits to check the namespace limits.
• Perform tests for the various validations mentioned above.

Verification:

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.