* Update the version of the checkout action (for GitHub Actions) in
`clang-format.yml` and `levelization.yml`.
* The previous version, v2, was raising deprecation warnings due to
its reliance on Node.js 12.
* The latest checkout action version, v3, uses Node.js 16.
* Per actions/runner-images#6002, ubuntu-18.04 is being deprecated. If
latest ever fails in the future, we'll need to fix the jobs anyway, so
catch it early.
* Use long option names
* Force clang-format to ubuntu-20.04 because LLVM 10 is not available for 22.04
* Add instructions to the workflow at the point where the failure would
occur, which is where someone experiencing a failure is most likely to
look. To keep things simple, the instructions are always printed. The
assumption is that if the job succeeds, nobody is likely to look
anyway.
* Provides the diff of the failure as an artifact, so the user can apply
it directly to their repo.
* Also update the levelization/README.md to clarify the levels a little
bit.
