chore: change Debian/Ubuntu instructions to use the deb822 format

[dhs-rec]

Fixes syncthing/syncthing#10495

[imsodin]

I opened the page locally, looks as one would expect:

Also ran all the commands on my debian laptop after removing my old syncthing.gpg and syncthing.list, to check there's no typos or similar. Got the expected stable and candidate versions after apt update with apt policy syncthing.

[monerostar]

Tested the filename behavior on Ubuntu 26.04 with apt 3.2.0, using a throwaway sourceparts dir so nothing real was touched.

With the stable-v2 block written to syncthing.source (singular) and the candidate block to syncthing.sources, apt only fetched the candidate channel. With syncthing.source alone, zero syncthing URIs were fetched. apt only picks up .list and .sources filenames in /etc/apt/sources.list.d, per the sources.list(5) man page.

So on modern Ubuntu the first block as written would be silently ignored, and users would get the candidate channel but not stable-v2. If Debian accepts the singular extension, that is a distro difference worth knowing. Suggest both blocks write to syncthing.sources.

[dhs-rec]

@monerostar, that's just a typo. Will fix...

[acolomb]

I'd say these instructions are a good step for modernization. But the new format is overly verbose and much harder to get right for copy-pasting.

Fortunately, we don't need to reinvent the wheel: There is https://tracker.debian.org/pkg/extrepo, which simplifies the whole procedure and relieves the user of having to understand and take responsibility for the security implications in adding all this code to their system. It shifts trust toward the maintainers of https://salsa.debian.org/extrepo-team/extrepo-data including measures to verify what you're getting matches what they publish. Key file management and repository source entry are handled in one step.

There are already entries syncthing, syncthing_stable_v2 and syncthing_candidate being maintained there, which point to this very website / repository.

I propose to add this as a more user-friendly alternative to the general-purpose, transparent deb822 style instructions. Basically:

sudo apt install extrepo
sudo extrepo enable syncthing_stable_v2
sudo install syncthing

This should work on roughly Ubuntu 22.04 (Jammy) and Debian 11 (Bullseye) and newer.

The text proposed in this PR should still be merged, with an explanation that it can be used as a fallback or when not wanting to delegate supply chain trust to yet another tool. I can assist with updating the text to have both versions available.

What do you think?

[dhs-rec]

Sounds good at a first glance, but since you mentioned Ubuntu: The extrepo-offline-data package seems to contain only Debian related repositories, even on Ubuntu.

[acolomb]

True, and it's the same situation for the online version: https://extrepo-team.pages.debian.net/extrepo-data

Nonetheless, it works fine on Ubuntu, I tested it on 26.04. Ubuntu does ship an /etc/debian_version file, which lists the corresponding Debian release, and that's what extrepo seems to use for mapping.