ci: add dependabot config for GitHub Actions

Enable dependabot for the "main" branch, letting it scan for outdated
GitHub Actions used in workflows on a weekly base.

Signed-off-by: Pino Toscano <ptoscano@redhat.com>

.github/dependabot.yml

@@ -0,0 +1,9 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    target-branch: "main"
+    commit-message:
+      prefix: "ci"