This saves time in CI spent performing the same source-level checks in
every build job. Checkstyle, PMD, and Spotless are now run once in the
"Lint and Format" job.
The -PskipPMD flag was replaced with a -PskipJavaFormat flag that
disables Checkstyle, PMD, and Spotless.
This reverts commit d068fb321f (#3420).
The 18.04 docker images now use GCC 8, which was the original reason for
updating. 20.04 uses a much more recent libc, so staying with 18.04
enables more Linux platforms to use the binaries.
