chore(buildtool): Auto-deprecate old Spinnaker versions (#43)

[spinnakerbot]

The publish_changelog function in the buildtool currently
publishes a new changelog for the newly-released Spinnaker version.
After publishing a patch version, the release manager needs to
manually edit the spinnaker.github.io repository to deprecate the
prior patch version, which is unnecessary manual toil.

This change updates the publish_changelog so that, in addition to
publishing a new changelog, it also deprecates the changelog
corresponding to the prior patch release. If this is the first
patch release of a minor version, it will short-circuit and will
not deprecate anything. (Maybe at some point we could update it
to deprecate the last patch release of the N-3 minor version but
for now that will stay manual.)

This also adds a newline at the end of each generated changelog,
as this always gets added when eventually deprecating it (whether
manually or via this script), increasing the diff.

Automated cherry pick of 1c94dcf into 1.17