[Settings] Delete update channels at shutdown

SettingsCache owns the stable and beta update channels as raw pointers, but
they have no parent and are never deleted. Each owns a QNetworkAccessManager,
so the test process leaves QtNetwork objects alive past QApplication teardown.

Delete the channels in the SettingsCache destructor so their network managers
are destroyed while the application object is still alive.

cockatrice/src/client/settings/cache_settings.cpp

@@ -50,6 +50,13 @@ SettingsCache &SettingsCache::instance()
     return *settingsCache; // returns a QT managed singleton reference
 }
 
+SettingsCache::~SettingsCache()
+{
+    for (ReleaseChannel *releaseChannel : releaseChannels) {
+        delete releaseChannel;
+    }
+}
+
 QString SettingsCache::getDataPath()
 {
     if (isPortableBuild) {

cockatrice/src/client/settings/cache_settings.h

@@ -99,6 +99,7 @@ private:
 
 public:
     SettingsCache();
+    ~SettingsCache() override;
     QString getDataPath();
     QString getSettingsPath();
     [[nodiscard]] QString getCachePath() const;