Update stage metadata to match reality

Author: soltysh

keps/sig-apps/2307-job-tracking-without-lingering-pods/kep.yaml

@@ -13,7 +13,7 @@ approvers:
 - "@janetkuo"
 
 # 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