Update the list of backends in the usr.sbin.cups AppArmor profile (Bullseye edition)

1. Check that the list of backends we ship in /usr/lib/cups/backend are all listed in the (patched) /etc/apparmor.d/usr.sbin.cups:

backends shipped in the cups, cups-daemon, cups-filters-core-drivers and cups-filters packages should have ixr; note that some are hard-linked by /var/lib/dpkg/info/cups.postinst to the corresponding file in /usr/lib/cups/backend-available/
other backends should have Cx -> third_party

2. Create a ticket to do the same for Tails 6.x.

Original created by @intrigeri on 16745 (Redmine)

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