chore: update React examples to match with the new React guidelines

[charpeni]

Summary

beta.reactjs.org brings new guidelines for code examples. One notable change is that named function expressions are now preferred over arrow functions for top-level functions.

Updating all our React examples to match with the guidelines as well as removing React imports in favor of the new JSX transform. This should reduce some confusion caused by the disparity between Jest docs and React docs.

⚠️ Those changes weren't applied to the versioned_docs as they could be released in the next release.

Test plan

Examples in the docs are showing up nicely on next.

[codecov-commenter]

Codecov Report

Merging #12217 (f6aa020) into main (11d79ec) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main   #12217      +/-   ##
==========================================
- Coverage   67.51%   67.51%   -0.01%     
==========================================
  Files         328      328              
  Lines       17246    17246              
  Branches     5071     5071              
==========================================
- Hits        11644    11643       -1     
- Misses       5569     5570       +1     
  Partials       33       33              

Impacted Files	Coverage Δ
packages/expect/src/utils.ts	96.03% <0.00%> (-0.50%)	⬇️

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[SimenB]

Thanks for this @charpeni!

Can we remove the weird .react file name convention you comment on separately (in all versioned docs and examples/tests)? I think that change is 100% correct. Not as sure about making automatic runtime default (although probably a good idea), but easier to evaluate separately 🙂

[charpeni]

Sure thing. I reverted changes to .react.js file convention in 707f8b1 and I'll send this in a separate pull request later.

[SimenB]

thanks!

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