Support Ubuntu 22.04

[ledyba-z]

See: avif-community/cavif#2

• Build at 22.04 and drop 18.04 support. cavif#3
• ci: switch to ubuntu 22.04 build davif#1
• Drop 18.04 support

[bradh]

@ledyba-z I have proposed changes at #2 that will address part of this (hopefully most of the first and last points).

I am unsure what is intended for the second point. Can you please provide more guidance, or perhaps complete that part?

[bradh]

Oh. Does the second point possibly relate to update of workflows at https://github.com/avif-community/davif/blob/master/.github/workflows/build-debian-packages.yml and https://github.com/avif-community/davif/blob/master/.github/workflows/build-on-linux.yml ?