Forms and Data Handling: Use matchedData() To Read from req.body

[ZackHoang]

Because

Change req.body data access by accessing it through matchedData() rather than accessing it directly. Also add a note about why this is done and how req.query is not mutable in the current version of Express

This PR

• Use matchedData() to access req.body
• Add a note about req.query being unable to mutate

Issue

Closes #30190

Additional Information

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[ZackHoang]

@mao-sz Sorry that you had to fix my entire PR lol 🤣

[mao-sz]

Nothing to apologise for