Check charts versions and replace branch variables in Rakefile

This is needed for the release:single raketask. It adds one fail-safe in case the charts version is not in charts_versions.yaml, so if you run bundle exec rake "release:single[13.2] and the mapping doesn't exist, it will abort and tell you about the error.

It also replaces the branch variables in .gitlab-ci.yml with the respective stable ones, which further automates the 4th step in https://docs.gitlab.com/ee/development/documentation/site_architecture/release_process.html#2-create-an-image-for-a-single-version.