update stage to stable

Author: ivelichkovich

keps/sig-api-machinery/3716-admission-webhook-match-conditions/kep.yaml

@@ -30,7 +30,7 @@ see-also:
   - https://github.com/kubernetes/enhancements/pull/3694
 
 # The target maturity stage in the current dev cycle for this KEP.
-stage: beta
+stage: stable
 
 # 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