Encourage use of K8s secrets

What does this MR do?

Originally, this MR was to fix the configuration sample for using Azure Blob Storage as Cache for a Kubernetes runner.

Since the newer MR !3300 (merged) (that was opened after this 🤷) was merged first, this MR is more about encouraging the use of K8s secrets. But it is also returns the documentation on the use of Azure Blob Storage as Runner Cache to a consistent state again.

Why was this MR needed?

1. The other MR !3300 (merged) removed the secret in the example code, but not from the text following the example. Thous it left the documentation in an inconsistent state. This MR rectifies this.
2. Storing secrets (such as Azure Storage access keys or GitLab Runner (Registration) Tokens) in plain configurations files should IMO not be favored over storing these secrets in K8s secrets. Also, K8s secrets are very simple to manage and aren't a high barrier at all.

What's the best way to test this MR?

Compare it to the Helm Chart reference https://gitlab.com/gitlab-org/charts/gitlab-runner/blob/main/values.yaml.

What are the relevant issue numbers?

None, not sure though.