Fix dependabot config to ensure allow is an array

[jmhbnz]

My last pr didn't quite get the dependabot config right, allow is supposed to be an array, refer: https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#allow

[spzala]

Ah, YAML's tricky sometimes :) Thanks for fixing it @jmhbnz

[ivanvc]

It sounds like linting YAMLs would be a good improvement. Running yamllint against the old dependabot.yml file yields:

.github/dependabot.yml
  9:9       error    wrong indentation: expected 6 but found 8  (indentation)
  10:9      error    duplication of key "dependency-type" in mapping  (key-duplicates)