Adjust Namespaces::Storage::RootSize cache keys

What does this MR do and why?

Switch to using Namespace::RootStorageStatistics#cache_key when caching in the Namespaces::Storage::RootSize#current_size method, so that the cache will expire when the related statistics have been updated.

This means the current_size cache will be busted when the root statistics are refreshed, but will fall back to a 10 min cache TTL as before if not.

Refs https://gitlab.com/gitlab-org/gitlab/-/issues/364124

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