Allow users to access agent through `user_access` keyword

What does this MR do and why?

This MR enables kas_user_access and kas_user_access_project feature flags by default, which have already been enabled on gitlab.com for a while.

Documentation is updated in Doc: Enable feature flags for Dashboard for Kub... (!123483 - merged).

Related [Feature flag] Clean up `kas_user_access` (#391201 - closed) [Feature flag] Clean up `kas_user_access_project` (#391211 - closed)

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