Include purchased storage in combined banner

What does this MR do and why?

Combined storage and free user limit banners should include any purchased storage as part of the limit.

This MR modifies the banner to use the dashboard limit method which includes additional storage.

These banners are feature flagged (disabled) hence no changelog.

Refs point 2 in https://gitlab.com/gitlab-org/gitlab/-/issues/432505#ask:

Confirm the limit states in this combo user limits and storage limits pre-enforcement banners is not just 5 GiB but 5 GiB + any additional storage purchased by the customer

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before (no additional storage)	After (2 MiB additional storage)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.