CI: Add dependabot to keep our CI actions up-2-date. (#8)

Author: aentinger

.github/dependabot.yml

@@ -0,0 +1,13 @@
+# See: https://docs.github.com/en/code-security/supply-chain-security/configuration-options-for-dependency-updates#about-the-dependabotyml-file
+version: 2
+
+updates:
+  # Configure check for outdated GitHub Actions actions in workflows.
+  # Source: https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/assets/dependabot/README.md
+  # See: https://docs.github.com/en/code-security/supply-chain-security/keeping-your-actions-up-to-date-with-dependabot
+  - package-ecosystem: github-actions
+    directory: / # Check the repository's workflows under /.github/workflows/
+    schedule:
+      interval: daily
+    labels:
+      - "topic: infrastructure"