Update docs about Synapse from Debian's repos

Synapse was removed from Debian bookworm at the end of May on the grounds that this project could not guarantee code reviews of CVE backports. There are also open security issues in sid, although it technically remains installable there.

The documentation offering Debian's own repo as an installation option as "maintained" is therefore out of date and should be updated, perhaps merging it with the Ubuntu section below until the situation changes.

(I think this situation is unfortunate because installing an additional repo is a hurdle to usage, and 1.78 could still have been a reasonable support target, but this issue is just about updating the documentation.)