ci: let Dependabot keep the workflow actions current

Nothing proposes updates to the workflow actions: several still declare
the Node 20 runtime, which runners already force to Node 24 with a warning
on every run. Dependabot now checks them monthly and groups all action
updates into one pull request.

.github/dependabot.yml

@@ -0,0 +1,13 @@
+version: 2
+updates:
+  # Workflow actions are pinned to commit SHAs; this keeps the pins current
+  # (new releases, runtime moves such as Node 20 -> 24) in one PR a month.
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: monthly
+    groups:
+      actions:
+        patterns: ["*"]
+    commit-message:
+      prefix: ci