cmd/gopherbot: allow backporting to the upcoming Go release
cmd/gopherbot: allow backporting to the upcoming Go release

This change gives gopherbot support to backport to the next major
release (current release + 1). Previously, attempting a backport to
a future release would create backports for the last two major
Go releases. These changes will not affected "please backport" comments
which do not indicate a backport number, as gopherbot should not open
an issue for the future release without being explicitly asked to do so.

Fixes golang/go#27040

Change-Id: I0096680c2ce2e164e6d80203bcf257dadaa97138
Reviewed-on: https://go-review.googlesource.com/131077
Reviewed-by: Filippo Valsorda <4f40cda291c5f9634e1affd3db44947af61f705c@golang.org>

golang/build,golang/build,golang/build,golang/build