web: preferentially use node 16 to compile

[midgleyc]

Node has released 16 as the latest LTS version, leaving 14 as the maintenance version. Preferentially use node 16 to compile.

In theory, it should be more performant. In practice, I can't really see a difference.

Nonetheless, it's worth doing, and I expect our dependent libraries to stop supporting node 12 as time goes by.

[Herschel]

Does this take care of #2724 as well?

[midgleyc]

I expect #2724 was fixed when archiverjs/node-archiver#491 was fixed, which was probably version 5.3.0 (though the issue is several dependencies deep).

Looks like we've hit the same problem with "Required" builds as before: can "Test Node.js 12 / Rust stable / ubuntu-latest" and "Test Node.js 12 / Rust stable / windows-latest" be made non-required (and after this possibly the same builds but with 16 be made required)

[Herschel]

Yep, I'll force merge this PR, and the new build steps will appear in the repo settings, at which point I can switch the new ones to required.

[Herschel]

Thank you!