Draft: Make projects limit field readonly when namespace limit enabled

What does this MR do and why?

Screenshots or screen recordings

Group's settings:

before	after

Project's settings:

before	after

How to set up and validate locally

Before we start testing:

• ::Gitlab::CurrentSettings.update(automatic_purchased_storage_allocation: true)
• ::Gitlab::CurrentSettings.update(check_namespace_plan: true)
• Make sure you're simulating SaaS

• As root access a group's settings page, then scroll to "Repository size limit (MiB)"
• As root access a project's settings page, then scroll to "Repository size limit (MiB)"

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 #364279