Checkmk Werk 15560: Drop support for Debian-9

ID: 15560 Title: Drop support for Debian-9 Component: Site Management Level: 1 Class: Bug fix Version: 2.2.0b6 Checkmk won't be built anymore for Debian-9 (stretch) from 2.2.0 and upwards as Debian-9's LTS support ended on June 30, 2022: https://wiki.debian.org/LTS The support from our side was already dropped during the beta phase with <tt>2.2.0b5</tt>.