all: update codereview.cfg for gopls-release-branch.0.20

This is an automated CL which updates the codereview.cfg.

For golang/go#74655

Change-Id: I2e456ae747315d0b4d984dca75fd73a005c6cf27
Reviewed-on: https://go-review.googlesource.com/c/tools/+/688495
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Auto-Submit: Gopher Robot <gobot@golang.org>
Reviewed-by: Hongxiang Jiang <hxjiang@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
diff --git a/codereview.cfg b/codereview.cfg
index 3f8b14b..ae541a1 100644
--- a/codereview.cfg
+++ b/codereview.cfg
@@ -1 +1,3 @@
 issuerepo: golang/go
+branch: gopls-release-branch.0.20
+parent-branch: master