Revert "Undici"

[Rich-Harris]

Gah — we might need to revert #5117. undici doesn't implement multipart/form-data parsing, unlike node-fetch. Since that's the default encoding, this means that forms are basically broken as a result of the switch. We just didn't have any tests covering it.

Related issue: nodejs/undici#974

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[Rich-Harris]

closing in favour of #5292