version: update MinClusterVersion to v2.2.0

This is the preparation for bumping to v2.3.0-alpha

version/version.go

@@ -26,7 +26,7 @@ import (
 
 var (
 	// MinClusterVersion is the min cluster version this etcd binary is compatible with.
-	MinClusterVersion = "2.1.0"
+	MinClusterVersion = "2.2.0"
 	Version           = "2.2.0+git"
 
 	// Git SHA Value will be set during build