Added documentation on how to handle the updated repository name in current Debian testing (see https://www.reddit.com/r/debian/comments/ca2fvn/debian_testing_fails_to_upgrade_now/?utm_source=share&utm_medium=web2x and https://www.mail-archive.com/debian-security@lists.debian.org/msg41223.html for details on why this is required).
Fixes QubesOS/qubes-issues#5260
QubesOS/qubes-issues#4693