[release-branch.go1.12] doc: add CherryPickApproved filter to Release History links

Not all closed issues in a given minor milestone are included in that
release, only the ones that have been labeled as CherryPickApproved are.

Update the links to the GitHub issue tracker to include a filter on the
CherryPickApproved label, so that the default view shows only the
backports that were included in a given release. This should more useful
to most people than seeing all backports (considered and approved).

Do this only for Go 1.9.1 and newer releases, as that is when we started
using the CherryPickCandidate and CherryPickApproved labels.

Updates #35988
Fixes #36002

Change-Id: I51e07c1bc3ab9c4a5744e8f668c5470adf78bffe
Reviewed-on: https://go-review.googlesource.com/c/go/+/210118
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>