ci: add Dependabot config for GitHub Actions maintenance (#110)

Automatically monitor the actions used in the CI workflows and open weekly
update PRs when newer versions are released.

Author: lfoppiano

.github/dependabot.yml

@@ -0,0 +1,12 @@
+version: 2
+updates:
+  # Keep GitHub Actions used in the workflows up to date
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    labels:
+      - "dependencies"
+      - "github-actions"
+    commit-message:
+      prefix: "ci"