Use Dependabot to keep GitHub Actions up to date

Configure Dependabot to send automated PRs to update the version numbers
in `.github/workflows/*.yml` when new versions of actions are released.

See:

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

Author: seanh

.github/dependabot.yml

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