Demote some Vale warnings to suggestions

[jdbaldry]

Anything with regular false positives.

• Grafana.GoogleOxfordComma
  • https://github.com/grafana/website/pull/19030/files#r1538318854
  • TODO compare the implementation with https://github.com/redhat-documentation/vale-at-red-hat/blob/main/.vale/styles/RedHat/OxfordComma.yml