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...