Restore React and ReactDOM umd builds

[youknowriad]

In 6.6 we tried moving away from the deprecated React UMD builds, the issue we faced is that there's a warning that is triggered on the console because we're not using a separate impact for createRoot. (SCRIPT_DEBUG true and npm run build)

Warning: You are importing createRoot from "react-dom" which is not supported. You should instead import it from "react-dom/client".

This warning has been removed in React 19 though along with the removal of the UMD builds, so we should be able to revert this PR when we upgrade to React 19.

Trac ticket: https://core.trac.wordpress.org/ticket/61324

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Props youknowriad, mamaduka.

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[github-actions]

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[Mamaduka]

@youknowriad, can you share testing instructions for the core? From our Slack conversation, I remember that some changes were required for wp-env.

[youknowriad]

Yes, so to reproduce the issue on core, you have to use the "build" folder in your environment so

• in .env file, you can change the LOCAL_DIR to build instead of src
• run npm run build
• run npm run env:start to launch the environment

you should be able to see the issue in trunk but not in this PR.

[youknowriad]

Commit https://core.trac.wordpress.org/changeset/58775