Migrate to PHP CodeSniffer 3
Stretch has phpcs 2.7.1, but Buster brings 3.4.0, so we need to adapt our sniffs to the new version, and change the docker image to use buster instead of Stretch.
This is needed to suppport newer versions of PHP.
Upgrade guide: https://github.com/squizlabs/PHP_CodeSniffer/wiki/Version-3.0-Upgrade-Guide