Update the box Ubuntu18->Ubuntu20

[arm4b]

Currently the build is based on Ubuntu18 iso which will be dropped in st2 v3.9.0
Switch to U20 box, update https://github.com/StackStorm/packer-st2/blob/master/st2.json and other places.

We run full packer build pipeline in Github Actions.

[arm4b]

If anyone from @StackStorm/contributors and @StackStorm/maintainers is interested to play with it.