Restrict activating members when on limit

What does this MR do and why?

Part of https://gitlab.com/gitlab-org/gitlab/-/issues/352638. In the linked issue we want to toggle existing memberships of a user for free namespaces. However, we also need to validate that there are still seats left (when the feature flag is activated for the group).

This MR adds the logic to not being able to activate a member when a namespace is at the limit. No changelog because it's behind a feature flag free_user_cap.

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