change: deprecate link to Misskey Forum in favor of GitHub Discussions (#10212)

see: https://forum.misskey.io/d/112-misskey-forum-to-end-service-at-the-end-of-this-month

CONTRIBUTING.md

@@ -15,7 +15,7 @@ Before creating an issue, please check the following:
 - To avoid duplication, please search for similar issues before creating a new issue.
 - Do not use Issues to ask questions or troubleshooting.
 	- Issues should only be used to feature requests, suggestions, and bug tracking.
-	- Please ask questions or troubleshooting in the [Misskey Forum](https://forum.misskey.io/) or [Discord](https://discord.gg/Wp8gVStHW3).
+	- Please ask questions or troubleshooting in ~~the [Misskey Forum](https://forum.misskey.io/)~~ [GitHub Discussions](https://github.com/misskey-dev/misskey/discussions) or [Discord](https://discord.gg/Wp8gVStHW3).
 
 > **Warning**
 > Do not close issues that are about to be resolved. It should remain open until a commit that actually resolves it is merged.