CHANGELOG.md: set version to 4.3.0 (2025-05-28)

Author: greenrobot

objectbox/CHANGELOG.md

@@ -1,6 +1,7 @@
-## latest
+## 4.3.0 (2025-05-28)
 
 * Update ObjectBox database for Flutter Linux/Windows, Dart Native apps to [4.3.0](https://github.com/objectbox/objectbox-c/releases/tag/v4.3.0).
+  This includes significant improvements to ObjectBox Sync like raising the maximum messages/transaction size.
 * Update ObjectBox database for Flutter Android apps to 4.3.0.
   If your project is [using Admin](https://docs.objectbox.io/data-browser#admin-for-android), make
   sure to update to `io.objectbox:objectbox-android-objectbrowser:4.3.0` in `android/app/build.gradle`.