Change `k8s-workloads/gitlab-com` charts to use in-repo vendored charts

This issue is an output from the discussion issue at #1959 (closed)

We need to remove helm-git as it's holding us up on upgrading our other Kubernetes components due to bugs and lack of maintenance.

In #1959 (closed) alternative solutions were discussed, but the chosen solution is to vendor all charts we use inside the gitlab-com repo directly, for speed, readability, and to lessen external dependences. We will just vendor charts directly into the repo (with dependencies) by straight file copy/add, similar to how tanka works.

We also want to add back the ability to do chart bumps per environment, instead of globally.

Work to be done

• Move raw chart to be vendored in repo directly (no need to use git-subtree) under charts/raw
• Develop automation in k-ctl to run helmfile deps upon execution Not needed as we will vendor everything, including deps
• Develop a tool in bin to allow us to easily do chart bumps. e.g. ./bin/chart-bump gstg SHA1234
• Move gitlab chart to be vendored in repo directly under charts/gitlab/${environment}
• Move gitlab-runner chart to be vendored in repo directly via under charts/gitlab-runner/${environment}
• Review the CI job build_chart to either be inside normal jobs, change from artifacts to cache, or something else (as needed)
• Drop helm-git plugin from CI image