Fix security mirror URL generated by apt-setup generator script

Since Debian changed the security mirror URL (as of bullseye), the related Debian Edu apt-setup generator script (70debian-edu-install) prevents the Debian installer apt-setup mechanism to write the security mirror entry also in case of an existing Internet connection.

The Debian Edu apt-setup generator script is useful in case of completely offline installations (using the BD ISO image). The manual informs about the missing security mirror entry in this case, see: https://wiki.debian.org/DebianEdu/Documentation/Bullseye/Installation#A_note_on_USB_flash_drive_.2F_Blu-ray_disc_image_installs

Fix this by pointing the generator script to the correct URL.