Fix serialization warning on Unity 6.3 (#1107)

* Fix "The type MixedReality.Toolkit.Core MixedReality.Toolkit.Experimental.BubbleChildHoverEvents is being serialized by [SerializeReference], but is missing the [Serializable] attribute. To suppress this warning add [Serializable] or use [MakeSerializable]."

* Update CHANGELOG.md

Author: keveleigh

org.mixedrealitytoolkit.core/CHANGELOG.md

@@ -2,6 +2,12 @@
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
+## Unreleased
+
+### Fixed
+
+* Fixed "The type MixedReality.Toolkit.Core MixedReality.Toolkit.Experimental.BubbleChildHoverEvents/TrickleChildHoverEvents/BubbleChildSelectEvents/TrickleChildSelectEvents is being serialized by [SerializeReference], but is missing the [Serializable] attribute." on Unity 6.3. [PR #1107](https://github.com/MixedRealityToolkit/MixedRealityToolkit-Unity/pull/1107)
+
 ## [3.3.0] - 2025-11-12
 
 ### Changed