Fixes NonOwnerAlertComponent auth check

What does this MR do and why?

Fixes NonOwnerAlertComponent auth check to ensures that only guests and above can view the component via their membership of the root namespace.

Previously to this MR, anyone viewing a public group would have passed the read_ability check and have seen the banner:

[1] pry(main)> Ability.allowed?(nil, :read_group, Group.public_only.first)
=> true

The banner cannot currently be viewed by users - it's a banner displayed to customers who are over both the storage and user limits, but storage limits have not been enabled yet.

This will be false until this feature flag check is true

Refs #431297 (closed)

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