Update docs about Synapse from Debian's repos

This issue has been migrated from [#16214](https://github.com/matrix-org/synapse/issues/16214).

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[Synapse was removed from Debian bookworm at the end of May](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1036806) on the grounds that this project could not guarantee code reviews of CVE backports. There are also [open security issues in sid](https://tracker.debian.org/pkg/matrix-synapse), although it technically remains installable there.

The documentation [offering Debian's own repo as an installation option](https://matrix-org.github.io/synapse/latest/setup/installation.html#downstream-debian-packages) as "maintained" is therefore out of date and should be updated, perhaps merging it with the Ubuntu section below until the situation changes.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.)

Read the downstream Debian packages section in setup/installation.html and compare its "maintained" wording with the linked Debian bug and tracker status. Update that documentation, including its relationship to the Ubuntu section, so the installation guidance accurately reflects Debian's current support status.

Written by the indexing model from the issue text.