CI: Add dependabot workflow to update github actions

This is based on the recommendation at https://github.blog/changelog/2023-01-18-code-scanning-codeql-action-v1-is-now-deprecated/#can-i-use-dependabot-to-help-me-with-this-upgrade

.github/dependabot.yml

@@ -0,0 +1,10 @@
+# Set update schedule for GitHub Actions
+
+version: 2
+updates:
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      # Check for updates to GitHub Actions every week
+      interval: "weekly"