Charts: Use a specific version of kubeconform Helm plugin

What does this MR do and why?

We should be using a specific version of the kubeconform Helm plugin, both because it's better for stability and because it prevents certain Docker build cache issues that would prevent the plugin from being updated when we want it to be. This just adds a version string to the installation.

Checklist

• (If applicable) Add patches if required for upgrading Ruby version in https://gitlab.com/gitlab-org/gitlab-build-images/-/tree/master/patches/ruby.