Svelte 4: drop Node 8 support

[benmccann]

Describe the bug

We're stuck on old versions of Mocha and Puppeteer since we're running Node 8 on the CI. We should drop Node 8 support when we have a Svelte 4 one day and then upgrade Mocha and Puppeteer

Reproduction

We might want to bump the engines field in package.json

Logs

No response

System Info

`master`

Severity

annoyance

[Conduitry]

We should drop Node 10 support too.

Node 12 is going to be EOL at the end of next April, so that might end up being on the table as well.

[stalkerg]

I suppose we can do it even before Svelt 4, at least for Node 8.

[bluwy]

Changing the supported node version is a breaking change and has to be done in a major release.

[benmccann]

done here: #8482