The Fetch spec has added the `Set-Cookie` header to forbidden request header names. See https://github.com/whatwg/fetch/pull/1453.
<rdar://problem/95811508>
Pull request: https://github.com/WebKit/WebKit/pull/2756
Committed 253325@main (593162b00deb): <https://commits.webkit.org/253325@main> Reviewed commits have been landed. Closing PR #2756 and removing active labels.