Disable auto-update check on unsupported versions of operating systems

From time to time on our user support channels we receive feedback from users on unsupported versions of operating systems (usually from users of macOS 10.11 or less) that the auto-update mechanism in Tor Browser continuously prompts for a check and tries to unsuccessfully update the browser.
I believe we can set a minSupportedOSVersion in the update xml files to disable these prompts (thanks to gk for this information!)

Thanks!