Upgrades from Debian 10 "Buster" directly to Debian 12 "Bookworm",
skipping Debian 11 "Bullseye", fail with apt erroring on:

    libcrypt.so.1: cannot open shared object file

This is an intentional OpenSSL transition as described in
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=993755

Hence, clean away such tests.

Also other minor cleanups in salsa-ci.yml.