Improve the pubsubbeat deployment

Our Pubsubbeat deployment needs some love:

• move the Helm chart into https://gitlab.com/gitlab-com/gl-infra/charts/
• implement proper image tagging in https://ops.gitlab.net/gitlab-com/gl-infra/pubsubbeat-image and enable Renovate for it
• revamp the chart: bring up to community standards, add documentation for Helm values, use image tags...