Migrate `k8s-workloads/plantuml` to `gitlab-helmfiles` repo

The plantuml service currently lives in it's own project unnecessarily: https://gitlab.com/gitlab-com/gl-infra/k8s-workloads/plantuml. This was due to the limitations of what k-ctl provided us when we were first configuring Kubernetes at GitLab. Now that we've introduced helmfile to deploy our GitLab services, we can now cleanly take this project and move it into the gitlab-helmfiles project: https://gitlab.com/gitlab-com/gl-infra/k8s-workloads/gitlab-helmfiles

• Discontinue CI on https://ops.gitlab.net/gitlab-com/gl-infra/k8s-workloads/plantuml
• Configure helmfile in https://gitlab.com/gitlab-com/gl-infra/k8s-workloads/gitlab-helmfiles to deploy the chart https://gitlab.com/gitlab-org/charts/plantuml
• Deprecate the old plantuml repositories on .com and the ops instance
• Update documentation as necessary