api: default to V3 capability

Fixes #7154

etcdserver/api/capability.go

@@ -33,9 +33,7 @@ var (
 	plog = capnslog.NewPackageLogger("github.com/coreos/etcd", "etcdserver/api")
 
 	// capabilityMaps is a static map of version to capability map.
-	// the base capabilities is the set of capability 2.0 supports.
 	capabilityMaps = map[string]map[Capability]bool{
-		"2.3.0": {AuthCapability: true},
 		"3.0.0": {AuthCapability: true, V3rpcCapability: true},
 		"3.1.0": {AuthCapability: true, V3rpcCapability: true},
 		"3.2.0": {AuthCapability: true, V3rpcCapability: true},
@@ -49,7 +47,10 @@ var (
 )
 
 func init() {
-	enabledMap = make(map[Capability]bool)
+	enabledMap = map[Capability]bool{
+		AuthCapability:  true,
+		V3rpcCapability: true,
+	}
 }
 
 // UpdateCapability updates the enabledMap when the cluster version increases.