Connection between oauth2-proxy and kubernetes-dashboard is insecure

--ssl-upstream-insecure-skip-verify

This should be replaced by a durable certificate on the dashboard and verification from the proxy. Might be a little tricky because the feature is missing from oauth2-proxy (https://github.com/oauth2-proxy/oauth2-proxy/issues/1901), but we might be able to inject our CA into the system's trust store.