Review and improve our checkstyle.xml config

The idea is to check the full list of checks at https://checkstyle.sourceforge.io/checks.html and make a proposal on the forum for suggested changes to our config. We haven't done this for a long time and checkstyle has evolved (more checks, more configs). We also now accept annotations for exclusions, which could change a bit our strategy.

Note that for all checks that we don't use, we should have a comment in our checkstyle.xml file to explain why we don't want to use it.