Update special-casing of feature/buster

E.g. in auto/scripts/utils.sh, auto/scripts/apt-mirror, and wiki/src/contribute/release_process.mdwn. In most places, s/buster/bullseye/ should be fine.

Feature Branch: https://salsa.debian.org/tails-team/tails/merge_requests/32, bugfix/misc-buster+force-all-tests

Related issues

Blocks #16209

Original created by @intrigeri on 16945 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information