[chore] bump undici for multipart form data support

[Conduitry]

This upgrades the version of undici used by Kit to one that support multipart form data, removes node-fetch as a dependency, and removes the hack in polyfills.js.

It also removes node-fetch as a (dev) dependency to adapter-node, because it was only being used in tests, which are currently not being run. We'll have to revisit this later anyway if we want to run those tests in CI, at which point we can hopefully rewrite them to use something other than node-fetch.

There is still node-fetch@2.6.7 in our tree via some other dependencies that we can't do much about, but this PR is able to remove the node-fetch@3 dependency.

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[changeset-bot]

🦋 Changeset detected

Latest commit: bda2832

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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