Update button for Public Groups (Free Tier) in Usage Quotas page

What does this MR do and why?

Related to #417976 (closed), this MR

1. updates the conditions to show the Add Seats button. In particular, it relies on the canAddSeats field
2. shows a new button Explore paid plans when the proper conditions apply

Screenshots or screen recordings

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

Condition	Expectation	Screenshot
Private Group
-- Free	Seats limit
Public Group
-- Free
	Explore paid plans
-- Premium
	Add seats
-- Ultimate
	Add seats
-- (OSS) Ultimate
	nothing

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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