Upgrade ubuntu version in GHA to 18.04

[AshesITR]

Fixes #456

[pvictor]

Thanks, indeed the action yaml is quite old.
What doyou think to use the last template https://github.com/r-lib/actions/blob/v2/examples/check-standard.yaml ?
Apparently version numbers are not hard written

Victor

[AshesITR]

The difference is that ubuntu-latest will be 20.04 instead of 18.04.
18.04 is older than 20.04, so using 18.04 should better ensure backward compatibility.
But I'm not sure this is much of a concern for shinyWidgets since it doesn't use any system libraries AFAIK.

Let me know if I should change

[pvictor]

I think ubuntu-latest is sufficient if it's okay for you

[AshesITR]

Of course. I've also upgraded the steps to the latest usethis template while at it.

[pvictor]

Thank you @AshesITR ! Really appreciate your contributions!