Fix use of wrong log function on heartbeat loss

[julianbrost]

Has to use the Warnw function as it passes additional zap attributes.

Before

2021-09-23T09:16:33.902+0200	WARN	icingaredis/heartbeat.go:149	Lost Icinga 2 heartbeat{timeout 8 60000000000  <nil>}

After

2021-09-23T09:29:30.913+0200	WARN	icingaredis/heartbeat.go:149	Lost Icinga 2 heartbeat	{"timeout": "1m0s"}