Updated changelog and package version for Netcode in anticipation of v3.1.0 release

Author: svc-netcode-sdk

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -6,7 +6,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 
 Additional documentation and release notes are available at [Multiplayer Documentation](https://docs-multiplayer.unity3d.com).
 
-## [Unreleased]
+## [3.1.0] - 2026-10-04
 
 ### Added