[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>