ffhb_11s.conf: add more test servers to domain_switch.connection_check_targets

- add some of our own VPN servers, so that the check also works for nodes
  that may only connect to the VPN servers
- remove one of the Google servers, to keep the number of servers low (the
  ping check is executed every minute and sends one ping packet to each IP,
  with a wait time of 10 seconds; so let's keep the total check time under
  60 seconds)

domains/ffhb_11s.conf

@@ -99,7 +99,9 @@
 		switch_time = 1653210000, -- Sun May 22 11:00:00 2022 CEST (this is just for testing)
 		connection_check_targets = {
 			'2001:4860:4860::8888',
-			'2001:4860:4860::8844',
+			'2a06:8782:ff00::f7', -- vpn01
+			'2a06:8782:ff02::e4', -- vpn02
+			'2a06:8782:ff02::e8', -- vpn07
 		},
 	},
 }