Drop compatibility with <= v2020 releases

The v2019 and v2020 release were built on Jenkins and hosted their Docker images in a dedicated registry. They also used an older version of ci-license-scan that did not support some command line flags.

For these reasons we had a couple of workarounds in place, but now that they reached the end of their cycles (v2020.7 has been released in November 2021) we can drop them.

This means that running a packaging pipeline on a v2020 branch will fail since it will not find the right Docker images.