Support multi-version backwards compatibility build for the monorepo

[dandhlee]

The monorepo for googleapis/google-cloud-python has multiple packages within the repository. I've relied on a split repository containing the git history using

sphinx-docfx-yaml/.kokoro/generate-docs.sh

Lines 71 to 77 in 273f5f1

	if [[ ${FORCE_GENERATE_ALL_TAGS} == "true" ]]; then
	# Grabs all tags from the repository.
	GITHUB_TAGS=$(git tag --sort=-v:refname)
	else
	# Grab the latest released tag.
	GITHUB_TAGS=$(git describe --tags `git rev-list --tags --max-count=1`)
	fi

to extract previous release info, however this may be more complicated for the monorepo.