The existing actions/checkout@v2 is causing Node v12 deprecation warnings
to be shown in GitHub Actions. v3 uses Node v16, which will stop the
warnings.
The CI scripts were using a PPA to get a backported version of uncrustify
on Ubuntu 20.04. However, this causes CI to intermittently fail due to
connection issues to launchpad.net or the key server.
Ubuntu 22.04 has a newer version of uncrustify removing the need for the
PPA. Ubuntu 22.04 is now in beta on GitHub actions, so it can be used.
Signed-off-by: David Lechner <david@pybricks.com>
