[bookworm] upgrading.dbk: add systemd early debug shell and credit Freedesktop.org for article

Add info to "4.1.4.2. Debug shell during boot using systemd" per the systemd early debug root shell.

Also in another commit, make clear to the reader that the Diagnosing Boot Problems article is from Freedesktop.org (matching 5.1.11. Deprecated components for bullseye). https://www.debian.org/releases/bullseye/amd64/release-notes/ch-information.en.html#deprecated-components

Recognize the early debug shell may be a bit too much information.