chore: drop node < 18

[bjohansebas]

I'll start from version 10, since we want to support HTTP2 directly in the tests (#16)

[bjohansebas]

Well, I think we can start from node 12 :)

[ctcpip]

looking at the http/2 PR as it is right now, it doesn't look like we'll actually need to drop support for old node yet. I mean, we can conditionally execute test behavior based on feature detection -- which we already do

[blakeembrey]

Ditto to @ctcpip's comments. I'm not against bumping the minimum node version, but I'd prefer to do it when we have major changes to the API.

I'm also sympathetic to it potentially being a maintenance burden for old versions but that's not what I've seen so far.

[bjohansebas]

If you don’t want to make the change to a major Node version yet, I’m perfectly fine with that