etcdserver/api: enable 3.4 capability

Signed-off-by: Gyuho Lee <leegyuho@amazon.com>

etcdserver/api/capability.go

@@ -40,6 +40,7 @@ var (
 		"3.1.0": {AuthCapability: true, V3rpcCapability: true},
 		"3.2.0": {AuthCapability: true, V3rpcCapability: true},
 		"3.3.0": {AuthCapability: true, V3rpcCapability: true},
+		"3.4.0": {AuthCapability: true, V3rpcCapability: true},
 	}
 
 	enableMapMu sync.RWMutex