Merge pull request kubernetes#123978 from deads2k/add-stefan

add stefan as apimachinery tech lead

Author: k8s-ci-robot

OWNERS_ALIASES

@@ -521,6 +521,7 @@ aliases:
     - sergeykanzhelev # Node
     - shaneutt # Network
     - soltysh # Apps, CLI
+    - sttts # API Machinery
     - tallclair # Auth
     - thockin # Network
     - xing-yang # Storage