Debian 13 silently resets custom mdadm RAID check schedule

Upgrading from Debian 12 to 13 makes the mdadm package remove the custom /etc/cron.d/mdadm file, saving it as .dpkg-bak, and enables the mdcheck systemd timers that were previously disabled. Custom array check frequency is replaced by the stock monthly schedule.

• mdadm 4.4-11 removes /etc/cron.d/mdadm and keeps edits as mdadm.dpkg-bak
• cron ignores filenames with dots, so the saved schedule never runs
• mdcheck_start and mdcheck_continue timers are enabled after the upgrade
• Restore via cron with AUTOCHECK=true or a systemd timer override