Upgrade to React v19 (#1195)

* migrate workspaces packages (except docs) to use React 19 by default

* restore subnav

* add more null checks for refs

* exclude one more flakey test

* fix race condition in axe tests

* disable parallel axe runs

Author: rezrah

.changeset/hungry-readers-switch.md

@@ -0,0 +1,15 @@
+---
+'@primer/react-brand': minor
+---
+
+Improved support for React v19.
+
+All components now have improved support for React v19 type signatures, in addition to continued backwards compatibility with React v18.
+
+As part of this change, the following has changed:
+
+- Refs as props are now handled correctly in `FAQGroup`, `ActionMenu`, `Bento`, `Button`/`ButtonGroup`, and other compound components.
+- Removed internal type casting on various components. This ensures that props are inferred correctly using the new JSX runtime in React v19.
+- Removed internal dependency on `react-is` for `Fragment` detection
+
+No changes to component behavior or presentation is expected as part of this change.