Upgrade testing image to Ubuntu 22.04

Similar to #190 (closed), it seems like it's time for another test image update, this time to Ubuntu 22.04 Jammy Jellyfish. I would propose to follow the same approach as with the upgrade from 18.04 to 20.04:

• Update the test image
• Denote in the docs what Utopia is tested against
• Do not (yet) upgrade the dependency enforcements

With Ubuntu 22.04 we can also upgrade to gcc 11.2. This also raises #211 again, and the question of whether enabling C++20 support is useful at this point?

People involved

@mackharald89 @JulianWeninger @tgaskin @blsqr @peanutfun @herdeanu