Turn on string ref deprecation warning for everybody (not codemoddable) (#25383)

## Summary
 
Alternate to https://github.com/facebook/react/pull/25334 without any
prod runtime changes i.e. the proposed codemod in
https://github.com/reactjs/rfcs/blob/createlement-rfc/text/0000-create-element-changes.md#deprecate-string-refs-and-remove-production-mode-_owner-field
would not work.

## How did you test this change?

- [x] CI
- [x] `yarn test` with and without `warnAboutStringRefs`