fix: Correctly handle proxies with undici

[PeterC89]

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Fixes #2259 🦕

This PR does a number of things:

• Removes redundant request options
• Removes old http-proxy-agent based proxying as this isn't supported by native node fetch
• Adds undici's EnvHttpProxyAgent to support proxies using standard environment vars
• Bumps minimum node version to 20 as 18 is EOL

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[meiswjn]

Nice PR!
@chingor13 any ETA on this already? Thanks!

[kauppfbi]

Hey @chingor13, is there any update on this PR? Any plans to introduce and release it soon, or do you plan a different implementation for #2259?

[schulmak]

Any updates on this PR? We would really appreciate a merge if nothing blocks it!

[hmrc87]

@chingor13 heavily needed here

[ferrarimarco]

Hi @PeterC89. Can you please rebase against the latest main, and elaborate on the changes you're proposing? thanks.