Refactor Namespace#root_storage_size Method

What does this MR do and why?

Refactor Namespace#root_storage_size to check all the conditions for namespace storage enforcement so that the correct type of storage enforcement, RootSize or RootExcessSize, will always be returned.

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/369153

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.