Use bookworm as new default release + update release list

There's no point in suggesting EOL releases in the dialog(1) dialog any
longer, so let's drop lenny, squeeze, wheezy, jessie + stretch there.
While at it switch from X.Y to X version schema as we always install
the latest Debian (point) release and not the displayed X.0 one.

Use bookworm as the new default release so we get grml-debootstrap
accordingly into the bookworm freeze (see
https://release.debian.org/testing/freeze_policy.html for details).

Update the docs regarding support releases.