Add changelog entry for 7.3.0

Author: greglittlefield-wf

CHANGELOG.md

@@ -1,3 +1,8 @@
+## 7.3.0
+- Add new, opt-in React 18 JS files (see [README](https://github.com/Workiva/react-dart#html) for more info)
+   - The preexisting JS files that use React 17 are now deprecated, and will be removed in the next major version, 8.0.0.
+- Raise SDK constraint from `<3.0.0` to `<4.0.0`, run CI on Dart 3
+
 ## 7.2.0
 - Add Dart wrapper for React [lazy](https://react.dev/reference/react/lazy)