Drop support for Node 12 and 14 (no longer LTS)

[voxpelli]

Looking at the LTS schedule for Node.js one can see that there is less than six months until Node 12 stops being in LTS.

Considering #277 and the plenty of nice features in Node 14, I think we should target Node >=14 for the 16.0.0 release. Adding this issue to that milestone for now, please object if you feel its wrong.

[voxpelli]

I would opt for a new minimum of 14.18 to ensure support for the node: prefix for modules that ship within node.js itself, as suggested by @MylesBorins: https://twitter.com/mylesborins/status/1521619664723402753

I personally have opted for a slightly more liberal version range in my projects though:

^14.18.0 || >=16

[voxpelli]

Now it would be time to opt for >=16