Merge ~rodsmith/certification-docs:fix-jammy-build-2 into certification-docs:master

Fix another build problem on jammy, caused by a renamed fonts file.

The ttf-ubuntu-font-family package was renamed to fonts-ubuntu some time ago, but the former name hung around as an alias for a while, and so continued to work through 22.10. Recently, though, jammy has dropped that name, forcing a rename. I've tested this change on my desktop (running 21.10) and it builds fine locally. I've also checked for the existence of the fonts-ubuntu package on an Ubuntu 18.04 system, and it's present, so the package SHOULD build there, too, but I haven't tested a build on 18.04, or on jammy, for that matter.