KEP-3619: update the latest milestone to v1.31

Author: everpeace

keps/sig-node/3619-supplemental-groups-policy/kep.yaml

@@ -23,11 +23,11 @@ stage: alpha
 # The most recent milestone for which work toward delivery of this KEP has been
 # done. This can be the current (upcoming) milestone, if it is being actively
 # worked on.
-latest-milestone: "v1.28"
+latest-milestone: "v1.31"
 
 # The milestone at which this feature was, or is targeted to be, at each stage.
 milestone:
-  alpha: "v1.28"
+  alpha: "v1.31"
   beta: "v1.xx"
   stable: "v1.yy"