Drop node.js 12 support and bump target to es2020

[webmaster128]

This is what https://github.com/paulmillr/noble-hashes uses too and we want to use noble hashes.

I guess applications can transpile this down to their supported target lists anyways.

[webmaster128]

Node.js < 14 does not support the Nullish coalescing operator from es2020, i.e. we need to drop node 12 support first.

Logical nullish assignment (??=) requires node.js 15 (ES 12/2021).

[webmaster128]

Node.js 12 EOLed on 2022-04-30. We can stop supporting it with CosmJS 0.29.0 and move to es2020 in the next release.