Mention external tileset unload in changelog, update Native.

Author: kring

CHANGES.md

@@ -13,6 +13,10 @@
 - Added `ML_CesiumRVTBaseColorOverlay` for Overlaying the BaseColor of an RVT in your material.
 - Added support for enum structural metadata properties to `CesiumPropertyArray`, `CesiumPropertyTable`, `CesiumPropertyTableProperty`, `CesiumPropertyTexture`, `CesiumPropertyTextureProperty`, and `CesiumMetadataValue`.
 
+##### Fixes :wrench:
+
+- "External Tilesets" are now unloaded when they are no longer used. This will significantly reduce the growth of memory usage over time when using Google Photorealistic 3D Tiles and similar tilesets.
+
 In addition to the above, this release updates [cesium-native](https://github.com/CesiumGS/cesium-native) from v0.44.3 to v0.45.0. See the [changelog](https://github.com/CesiumGS/cesium-native/blob/main/CHANGES.md) for a complete list of changes in cesium-native.
 
 ### v2.13.3 - 2025-02-12