[ICU-621] Add ExperimentalGroupUnreadChannels setting (#8127)

* Add ExperimentalGroupUnreadChannels setting

* track ExperimentalGroupUnreadChannels in diagnostics

app/diagnostics.go

@@ -240,6 +240,7 @@ func (a *App) trackConfig() {
 		"enable_preview_features":                                 *cfg.ServiceSettings.EnablePreviewFeatures,
 		"enable_tutorial":                                         *cfg.ServiceSettings.EnableTutorial,
 		"experimental_enable_default_channel_leave_join_messages": *cfg.ServiceSettings.ExperimentalEnableDefaultChannelLeaveJoinMessages,
+		"experimental_group_unread_channels":                      *cfg.ServiceSettings.ExperimentalGroupUnreadChannels,
 	})
 
 	a.SendDiagnostic(TRACK_CONFIG_TEAM, map[string]interface{}{