From Debian 11 Bullseye onwards, debian uses $dist-security instead of
$dist/updates as distribution part in /etc/apt/sources.list.
Mark those distributions with the old-style subdirectory path with
"security-subdir" in distributions.conf.
Thanks to Paul Wise for the bug report (and hence reminding me of
this) and for the suggestions on how to implement this (even if I
didn't follow them for the sake of simplicity and no additional
dependencies).
Closes: #972749
2fba5cb90e