Add Dependabot configuration for GitHub Actions updates (#90)

Add a Dependabot configuration that checks once a week if the GitHub Actions are still using the latest version. If not, it opens a PR to update them.

It will actually open very few PRs, since we only have major versions specified (like v3), so only on a major v4 release it will update and open a PR.

See https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot

.github/workflows/dependabot.yml

@@ -0,0 +1,8 @@
+version: 2
+updates:
+  # Maintain dependencies for GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      # Check for updates to GitHub Actions every week
+      interval: "weekly"