Update kep.yam and PRR

Author: jpbetz

keps/prod-readiness/sig-api-machinery/4358.yaml

@@ -5,4 +5,6 @@ kep-number: 4358
 alpha:
   approver: "@deads2k"
 beta:
+  approver: "@deads2k"
+stable:
   approver: "@deads2k"

keps/sig-api-machinery/4358-custom-resource-field-selectors/kep.yaml

@@ -16,7 +16,7 @@ see-also:
   - "/keps/sig-api-machinery/95-custom-resource-definitions"
 
 # 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
@@ -27,6 +27,7 @@ latest-milestone: "v1.31"
 milestone:
   alpha: "v1.30"
   beta: "v1.31"
+  stable: "v1.32"
 
 # The following PRR answers are required at alpha release
 # List the feature gate name and the components for which it must be enabled