codereview.cfg: update for gopls-release-branch.0.12

Updates golang/go#59818

Change-Id: I486ccfb67d4c5c935f1db1fe8d7f9c16fda9d915
Reviewed-on: https://go-review.googlesource.com/c/tools/+/488335
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>

codereview.cfg

diff --git a/codereview.cfg b/codereview.cfg
index 3f8b14b..db37e78 100644
--- a/codereview.cfg
+++ b/codereview.cfg
@@ -1 +1,3 @@
 issuerepo: golang/go
+branch: gopls-release-branch.0.12
+parent-branch: master