[release-branch.go1.20] update codereview.cfg for release-branch.go1.20

Following go.dev/cl/334376.

Change-Id: I96be6379566b5bcc31e41bdd5f30946b06001153
Reviewed-on: https://go-review.googlesource.com/c/go/+/455896
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
diff --git a/codereview.cfg b/codereview.cfg
index 77a74f1..6e621c3 100644
--- a/codereview.cfg
+++ b/codereview.cfg
@@ -1 +1,2 @@
-branch: master
+branch: release-branch.go1.20
+parent-branch: master