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

[eps1lon]

Summary

Alternate to #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?

• CI
• yarn test with and without warnAboutStringRefs

[sizebot]

Comparing: d65b88d...6f24ce6

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	153.62 kB	153.62 kB	=	48.83 kB	48.83 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	155.55 kB	155.55 kB	=	49.47 kB	49.47 kB
facebook-www/ReactDOM-prod.classic.js	=	530.40 kB	530.40 kB	=	94.14 kB	94.14 kB
facebook-www/ReactDOM-prod.modern.js	=	515.66 kB	515.66 kB	=	91.95 kB	91.95 kB
facebook-www/ReactDOMForked-prod.classic.js	=	530.40 kB	530.40 kB	=	94.15 kB	94.14 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 6f24ce6