- Ubuntu focal is the lowest LTS release that we can support with only
distro packages (e.g. no rustup).
- Remove tsan from Cirrus (it's not working currently, and also not really
important).
- Remove Centos (it passes tests but I'm not sure it's worth adding; there
isn't even an official docker image for CentOS Stream).
The Dockerfiles had bitrotted some.
Get them passing again, add libpcre2-dev where we can so we aren't
hitting more servers than necessary, and reformat the bionic files so
they can share more of the same image.
