chore: rename form accept-charset attribute

[hyunbinseo]

accept-charset — Character encodings to use for form submission

The accept-charset attribute gives the character encodings that are to be used for the submission. If specified, the value must be an ASCII case-insensitive match for "UTF-8". [ENCODING]

https://html.spec.whatwg.org/multipage/forms.html#attr-form-accept-charset

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[Rich-Harris]

thank you!