Add Go 1.20 support

Now that we've released 1.19 support, filing this issue to track work related to [1.20 support](https://go.dev/blog/go1.20).

Release notes: https://go.dev/doc/go1.20

Developer guide for new version support in [wiki](https://github.com/gopherjs/gopherjs/wiki/Development:-New-Go-version-support-guide).