CI: Let dependabot update GH Actions (#305)

This PR adds a configuration to tell Dependabot to update the versions
used for some well-known GitHub Actions, such as `checkout`, in order to
avoid deprecation warnings when those get too old.
[Documentation](https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot)

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"