docs: remove gitter reference (angular#60970)

PR Close angular#60970

Author: thesmiler

README.md

@@ -156,7 +156,6 @@ Join the conversation and help the community.
 [X (formerly Twitter)]: https://www.twitter.com/angular
 [bluesky]: https://bsky.app/profile/angular.dev
 [discord]: https://discord.gg/angular
-[gitter]: https://gitter.im/angular/angular
 [stackoverflow]: https://stackoverflow.com/questions/tagged/angular
 [youtube]: https://youtube.com/angular
 [meetup]: https://www.meetup.com/find/?keywords=angular