Upgrading to React 19

Purpose

This issue serves to track the ongoing work to add support for React 19.

Tasks

Tasks to do (some might need to be broken down further):

• Disable the React Native unit tests CI build: #71337
• Ensure we’re still not publishing new releases of the React Native packages (aztec, bridge, and editor)
• Update documentation to make it clear that the RN implementation is paused and may be broken in trunk.
• Update types to be compatible with React 19: #71374
• Fix InserterListbox rendering for React 19: #71410
• Edit Post: Check for meta box container before adding constraints: #71423
• Remove deprecated react-test-renderer dependency #71422
• Deps: Bump framer-motion to v11.15.0 #71442
• Handle UMD builds that were removed in React 19 #74343
• Update Emotion for React 19 compat #75324
• Update Testing Library for React 19 compat #75340
• Types: consistently use the React namespace #75499
• useRef: always supply initial value #75513
• Prefix usages of JSX namespaces with React.JSX #75508
• Update usage of RefObject types (React 19 compat) #75567
• useMergeRefs: migrate to TypeScript #75569
• NumberControl: merge two state reducers into one #75822
• BlockListBlock: fix crash when selectedProps are null #75826
• Add e2e test for sorting patterns #75823
• Finish the upgrade to Gutenberg Web to React 19, also using parts of #61521.
• Deprecate useRefEffect hook in favor of native useCallback ref cleanups.
• Update useMergeRefs to support ref callback cleanups.

Additional background

While breaking Gutenberg Mobile on trunk doesn't seem to be an ideal next step, it's a necessary one. The WP Mobile project is actively exploring paths forward for the block editor in the mobile apps, aiming to reduce complexity and improve alignment with the Gutenberg web project. There is currently little to no active work on the Gutenberg Mobile project. Therefore, to