Fix release CI stage failing due to Helm stable deprecation

CNCF deprecated the pulling of the Helm stable repo from kubernetes-charts.storage.googleapis.com (https://www.cncf.io/blog/2020/10/07/important-reminder-for-all-helm-users-stable-incubator-repos-are-deprecated-and-all-images-are-changing-location/). Since we're still using Helm v2.16, we can fix this by pointing the stable repo to the current location.

Relates to gitlab-org&4695