KEP-3619: added beta promotion in "Implementation History"

Author: everpeace

keps/sig-node/3619-supplemental-groups-policy/README.md

@@ -1216,6 +1216,7 @@ Major milestones might include:
 
 - 2023-02-10: Initial KEP published.
 - v1.31.0(2024-08-13): Alpha
+- v1.33.0(2025-04-23): Beta
 
 ## Drawbacks