Use `FormData` `submitter` parameter

[jenseng]

Summary

Fixes #29018

Rather than continuing to maintain/fix a partial polyfill (e.g. #28056), just pass the submitter to the FormData constructor, since it is now widely available. This ensures that the form data is populated consistently with a native form submission, and lets us remove a whole lot of code. This is a followup to this discussion over a year ago: https://github.com/facebook/react/pull/26674/files/5a7629ddb4f22072e37ba1ae581977ce03bdf288#r1178495317

Elaborating on that, if we're concerned about browsers that don't support it, it's important to note that:

• Many of them also don't support SubmitEvent.submitter, which means the existing approach won't work for them either
• The FormData submitter can easily be polyfilled (as can SubmitEvent.submitter) for developers that care about named submit buttons working correctly in old browsers

How did you test this change?

Updated an existing test to more fully cover submitter serialization, and ran yarn test and yarn test --prod successfully.