Stabilize reactFragments host node handle (#35642)

https://github.com/facebook/react/pull/34935 Introduced
`unstable_reactFragments` handle on DOM nodes to enable caching of
Observers.

This has been tested in production and is stable so it can be rolled out
with the Fragment Refs feature.

DiffTrain build for [9c0323e2cf9be543d6eaa44419598af56922603f](https://github.com/facebook/react/commit/9c0323e2cf9be543d6eaa44419598af56922603f)