doc: Update changelog.

Author: freezy

CHANGELOG.md

@@ -4,10 +4,11 @@
 
 ## Unreleased
 
-Built with Unity 2022.3.x
+Built with Unity 6.3
 
 ### Added
 
+- New threading model ([#552](https://github.com/freezy/VisualPinball.Engine/pull/552))
 - Free transformation ([#500](https://github.com/freezy/VisualPinball.Engine/pull/500))
 - Kinematic collisions ([#460](https://github.com/freezy/VisualPinball.Engine/pull/460))
 - Flipper tricks by nFozzy ([#436](https://github.com/freezy/VisualPinball.Engine/pull/436))
@@ -79,4 +80,4 @@ Built with Unity 2022.3.x
 - Layer names are correctly computed when importing a 10.6 file ([#291](https://github.com/freezy/VisualPinball.Engine/pull/291)).
 - Clear texture and material references that don't exist before writing (VP 10.7 behavior) ([#291](https://github.com/freezy/VisualPinball.Engine/pull/291)).
 - Bug in writing animation vertices which caused VP to hang when re-reading the file ([#291](https://github.com/freezy/VisualPinball.Engine/pull/291)).
-- A few bugs in drag point gizmos ([#246](https://github.com/freezy/VisualPinball.Engine/pull/246)).
+- A few bugs in drag point gizmos ([#246](https://github.com/freezy/VisualPinball.Engine/pull/246)).