Move to Ubuntu 20.04 (by April 2023)

[chrisjsewell]

#132 enables testing against this platform, and in-tandem with rolling this out I have been looking to make roles Ubuntu 20.04 compatible (or at least opening an issue to detail what needs to be added/changed)

Eventually then it would be desirable to build the QM releases on this platform.

Note Ubuntu 18.04 end of standard support is April 2023, see https://wiki.ubuntu.com/Releases, so I guess the goal would be by then

[ltalirz]

I see that some roles are already tested
Perhaps a checklist of the migration progress would be useful

P.S. Let's remember to revisit #144 once the transition to 20.04 is completed