Don't allow Syncthing to upgrade beyond the supports macOS version

[bumper314]

Come on folks, this is easy to prevent. Since v1.27.7 decided to drop support for versions of macOS prior to 12.7, all you have to do is add a <sparkle:minimumSystemVersion>12.7.0</sparkle:minimumSystemVersion> tag to your https://upgrades.syncthing.net/syncthing-macos/appcast.xml Sparkle feed to prevent unsupported versions of macOS from even seeing the Sparkle update popup.

No need for people to https://github.com/syncthing/syncthing-macos?tab=readme-ov-file#disable-automatic-update-checker

See https://sparkle-project.org/documentation/publishing/#minimum-system-version-requirements for more info

Related #216, #217, #218, #219

[kupietools]

This is seriously annoying. I've got a very old mac which still works perfectly, running so I have it running some administrative stuff, like Syncthing. Tonight it gave me the update notification so I updated. And now Syncthing doesn't work anymore, and this is on a headless machine I'm accessing over VNC and having all sorts of network problems, and I had to go find a compatible version, and once I got it installed and running it was stuck "offline" and I got sucked into troubleshooting, and what I thought was going to be a 30 second upgrade now has consumed about 2 hours of my time when I was in the middle of other things, and it's 3:30AM and I don't have a working install of Syncthing anymore, and really it was much easier when syncthing just worked, and I really would have prefered if it hadn't tricked me into destroying it.