This sets the workflow concurrency to 1 for all workflows. For PRs this means if you push an additional commit older jobs will be cancelled.
The documentation workflow already only runs on tags or merges to main. For this, we cancel previous runs if they are to the same destination (tag or main) but still prevent 2 jobs from running at once if they are spawned from different refs.
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.
