cmd/tsidp: update README with new repo location warning

Fixes: #17170

Signed-off-by: Remy Guercio <[email protected]>

cmd/tsidp/README.md

@@ -1,3 +1,6 @@
+> [!CAUTION]
+> Development of tsidp has been moved to [https://github.com/tailscale/tsidp](https://github.com/tailscale/tsidp) and it is no longer maintained here. Please visit the new repository to see the latest updates, file an issue, or contribute.
+ 
 # `tsidp` - Tailscale OpenID Connect (OIDC) Identity Provider
 
 [![status: community project](https://img.shields.io/badge/status-community_project-blue)](https://tailscale.com/kb/1531/community-projects)