Include Remaining Storage Size in Namespace Storage Email Notification

What does this MR do and why?

Adjust the namespace storage limit notification emails to include the remaining storage size in the email body.

This only applies to notifications for when the storage size is approaching the limit.

Emails for when the storage limit is exceeded are unchanged.

See this comment: !83222 (comment 899495560)

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/349682

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