Clear new_user_signups_cap when seat_control is not user_cap

What does this MR do and why?

Set namespace_settings.new_user_signups_cap to nil when seat_control is not :user_cap.

This will clear the number field for the Set user cap option when Seat controls are set to Open access in the group settings UI.

This is a follow up from !161368 (merged) to address some feedback there: !161368 (comment 2037596315)

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/470094

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Screen recordings

Before	After
BeforeCapSave-small	AfterCapSave-small