Always target v2019 from buster

The Apertis policy for the linux kernel differs from the one for most other packages, as we always try to track the latest LTS release for each release.

This means that only in a few cases we are aligned to what Debian provides: this happens in v2019/buster with 4.19 but for v2020 we already track the 5.4 LTS branch.

Let's then point the auto-merge machinery from buster to the v2019 channel as that's the only place where it should land.