Add support for setting the GitLab version globally

Closes: https://gitlab.com/charts/gitlab/issues/479

In addition to adding support for global.gitlabVersion. This change defaults to using the appVersion value from the chart. Which will be a field we are automatically updating.

Initially I was only planning for us to be updating the versioning information in the top level chart, but the subcharts wouldn't have access to their appVersion field then.

It also seems cleaner/easier for our automated versioning script to only need to update the Chart.yaml files, rather than needing to also update the global in the values.yaml.