ci: use dependabot to update GHA dep (#39)

.github/dependabot.yml

@@ -0,0 +1,16 @@
+version: 2
+
+updates:
+  - package-ecosystem: 'github-actions'
+    directory: '/'
+    schedule:
+      interval: 'monthly'
+    open-pull-requests-limit: 10
+    commit-message:
+      prefix: 'chore'
+    groups:
+      # Any updates not caught by the group config will get individual PRs
+      gha-low-risk:
+        update-types:
+          - 'minor'
+          - 'patch'