Make blocknote-headless compatible with React 17+

As reported by mflorea, we are currently explicitly importing React in blocknote-headless.

Copy of the conversation:
My feeling is that https://unpkg.com/@xwiki/cristal-editors-blocknote-headless@0.17.0/dist/index.es.js is not properly built. I mean, those React.createElement should not be there. Based on https://legacy.reactjs.org/blog/2020/09/22/introducing-the-new-jsx-transform.html this looks like the output of the old (classic) jsx transformation (from before React 17), but we're using React 19 so I'm assuming the new jsx transformation should be used, which produces something like _jsx('h1' instead of React.createElement('h1'

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