Bullseye (again)

Further changes for bullseye compatibility, which were applied during update of mail.fsmpi.

apt/defaults/main.yml

@@ -2,7 +2,7 @@
 # file: common/apt/defaults/main.yml
 
 apt_mirror: 'http://ftp.halifax.rwth-aachen.de/debian/'
-apt_mirror_security: 'http://security.debian.org/'
+apt_mirror_security: 'http://security.debian.org/debian-security/'
 apt_repos: ['main', 'contrib', 'non-free']
 apt_sets: ['', '-updates']  # ['-backports', '-proposed-updates']
 apt_src_enable: true