Most of our docker images are for an OS release which is past EOL. Most
are not checked in CI, which leads to more staleness. It's not
obvious which docker are expected to work and which are best-effort.
I've updated all of them in the past, which would be slightly easier
if we got rid of the redundancy.
Remove most unused ones for now, to reduce confusion and maintenance
effort. Some Ubuntu images are replaced by
docker/ubuntu-latest-lts.Dockerfile
docker/ubuntu-oldest-supported.Dockerfile
Leave around the fedora:latest and opensuse/tumbleweed:latest images
for now, though I don't think there's a reason to publish them in
build_docker_images until we add CI jobs.
We can add some images back (even past-EOL versions) but preferrably
with a documentted update policy (see next commit) and CI tests
(could be a nightly/weekly/pre-release check).
The new names are consistently formulated as commands.
`main` is not very descriptive, so change it to `test`, which is more
informative and accurate.
`rust_checks` are replaced be the more general `lint` in preparation for
non-Rust-related checks.
These changes were suggested in
https://github.com/fish-shell/fish-shell/pull/11918#discussion_r2415957733Closes#11922