New track for Node.js 16

[rvagg]

According to plan @ https://github.com/nodejs/snap/#adding-new-release-lines

1. Get a new track for the "node" Snap - in progress https://forum.snapcraft.io/t/track-request-for-node-16-fast-track-please/23969
2. Create a new branch named node16
3. Fix up snapcraft.yaml.sh for 16
4. Add 16 to the GitHub Actions matrix
5. Trigger manual build to get a push to Launchpad
6. Create Snap workflow in Launchpad for 16
7. Trigger manual builds in Launchpad for 16.0.0

I'd also like to try and make the switch to core20 for 16+. I should have tried this earlier for nightlies/master but #3 has me a bit concerned about trailing too far behind modern glibc for an LTS. See https://snapcraft.io/docs/base-snaps for documentation on bases. It may not be possible to use core20 for reasons mentioned in there but I'd like to give it a go.

[rvagg]

$ sudo snap refresh node --channel 16
node (16/stable) 16.0.0 from OpenJS Foundation (iojs✓) refreshed
$ node -v
v16.0.0

x64 is built at least, core20. But, core20 has meant we have to drop x86 from 16+. I think there might be a creative way around this by setting up a separate build that also publishes to the 16 track but using a different config, but that's getting kind of complicated and I think I'd prefer to defer it until we have user demand or someone else wants to contribute to making that happen.

I've pushed core20 to the edge (master) build too and dropped x86 from there as well.