Refactor Percent Namespace Storage Used

What does this MR do and why?

As noted in the issue here, we have two different functions on the backend for formatting and displaying the used namespace storage percentage.

This MR refactors so that we have only one function and use it consistently.

This MR also removes the RootSize#used_storage_percentage and RootSize#remaining_storage_percentage functions. remaining_storage_percentage is not used currently on master. used_storage_percentage is no longer used after this MR.

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/422103

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