Configure Dependabot updates for Bundler/Ruby

https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/about-dependabot-version-updates#supported-repositories-and-ecosystems

In future this will keep the Gemfile.lock up to date

.github/dependabot.yml

@@ -1,6 +1,11 @@
 # Basic set up
 version: 2
 updates:
+  # Maintain dependencies for Ruby
+  - package-ecosystem: "bundler"
+    directory: "/"
+    schedule:
+      interval: "daily"
   # Maintain dependencies for GitHub Actions
   - package-ecosystem: "github-actions"
     directory: "/"