(#1874) Disable updates on non-Windows, Linux or macOS platforms (#1914)

* (#1874) Disable update checking for unsupported platforms

As described #1874, only Windows, macOS and GNU/Linux are officially
supplied with builds. Thus checking for updates is unnecessary if we
are e.g. on FreeBSD, OpenBSD, illumos distros and possibly other
Operating Systems.

This fixes #1874 by ifdef-ing the 3 officially supported platforms
when checking for updates. Otherwise a debug warning will be emitted
and in the settings the checkbox for beta-updates is hidden and
replaced by a message, explaining the reason for why there are no beta
updates.

* Update CHANGELOG

In accordance with #1874

* (#1874) Move platform check into checkForUpdates

As discussed in #1914

* (#1874) Move check for supported OS to a seperate function.

As requested in #1914

src/singletons/Updates.cpp

@@ -232,6 +232,14 @@ void Updates::installUpdates()
 
 void Updates::checkForUpdates()
 {
+    if (!Version::instance().isSupportedOS())
+    {
+        qDebug()
+            << "Update checking disabled because OS doesn't appear to be one "
+               "of Windows, GNU/Linux or macOS.";
+        return;
+    }
+
     // Disable updates if on nightly and windows.
 #ifdef Q_OS_WIN
     if (Modes::instance().isNightly)