update kep status to implemented

the --redirect-container-streaming flag and feature gate will be removed in 1.24

Author: pacoxu

keps/sig-node/1558-streaming-proxy-redirects/kep.yaml

@@ -14,8 +14,8 @@ approvers:
   - "@Random-Liu"
   - "@derekwaynecarr"
 creation-date: 2019-12-05
-last-updated: 2021-05-11
-status: implementable
+last-updated: 2021-08-19
+status: implemented
 replaces:
   - "https://docs.google.com/document/d/1OE_QoInPlVCK9rMAx9aybRmgFiVjHpJCHI9LrfdNM_s/edit#heading=h.4yfjiw58o8d3"