Updating CI for release branch to remove Ubuntu trusty and add focal

This MR merges in a set of cherry-picked commits from master that update the CI to stop undertaking builds for Ubuntu 14.04 (trusty), which is now a long way out of it's LTS period, and to add Ubuntu 20.04 (focal) to the CI build system.

Not marking this as a WIP since it can be merged as is, however, @ccantwel @dmoxey, do you want to include the option of building CWipi in the next maintenance release? If so 807b55b2, 3900e8d7 and a8fc1bc0 should also be cherry-picked into the branch for this MR.