ICU-715 Change ExperimentalGroupUnreadChannels setting to allow for default on/off (#8211)

config/default.json

@@ -57,7 +57,7 @@
         "CloseUnusedDirectMessages": false,
         "EnableTutorial": true,
         "ExperimentalEnableDefaultChannelLeaveJoinMessages": true,
-        "ExperimentalGroupUnreadChannels": false,
+        "ExperimentalGroupUnreadChannels": "disabled",
         "ImageProxyType": "",
         "ImageProxyOptions": "",
         "ImageProxyURL": ""