fix(ui): use userId.hashCode for GradientAvatar colors (#2381)

Co-authored-by: xsahil03x <[email protected]>

Author: xsahil03x

packages/stream_chat_flutter/CHANGELOG.md

@@ -3,6 +3,8 @@
 🐞 Fixed
 
 - Fixed `.replaceMentions` not escaping special characters in the username.
+- Fixed `GradientAvatars` for users with same-length IDs would have identical
+  colors. [[#2369]](https://github.com/GetStream/stream-chat-flutter/issues/2369)
 
 ## 9.16.0