dillon1000/react

[react-dom] Remove `findDOMNode` from OSS builds (#28267)

In the next major `findDOMNode` is being removed. This PR removes the
API from the react-dom entrypoints for OSS builds and re-exposes the
implementation as part of internals.

`findDOMNode` is being retained for Meta builds and so all tests that
currently use it will continue to do so by accessing it from internals.
Once the replacement API ships in an upcoming minor any tests that were
using this API incidentally can be updated to use the new API and any
tests asserting `findDOMNode`'s behavior directly can stick around until
we remove it entirely (once Meta has moved away from it)