Change the severity of the etcd leader election check to warning

This is the first step towards the goal to only have metrics tracking
the overall health and performance of the component/cluster. For instance,
for etcd disruption scenarios, leader elections are expected, we should instead
track etcd leader availability and fsync latency under critical catergory vs leader
elections.

config/alerts

@@ -8,4 +8,4 @@
 
 - expr: increase(etcd_server_leader_changes_seen_total[2m]) > 0
   description: etcd leader changes observed
-  severity: critical
+  severity: error