gpui: Replace Mutex with RefCell for SubscriberSet (#30907)
`SubscriberSet` is `!Send` and `!Sync` because the `active` field of
`Subscriber` is `Rc`.
Release Notes:
- N/A
70b0c4d63dlaizy committed on 5/19/2025, 9:08:04 AM· committed by GitHubparent875d1ef