[dependabot] Update dependabot.yml to match other repos

Closes #60201

GitOrigin-RevId: cbea37a
Change-Id: Ife722a383f9676f4fefc63c97a35acde5a0f69d6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/411740
Reviewed-by: Alexander Thomas <[email protected]>
Reviewed-by: Devon Carew <[email protected]>

Author: athomas

.github/dependabot.yml

@@ -1,6 +1,13 @@
+# Dependabot configuration file.
+# See https://docs.github.com/en/code-security/dependabot/dependabot-version-updates
 version: 2
+
 updates:
-  - package-ecosystem: "github-actions"
-    directory: "/"
+  - package-ecosystem: github-actions
+    directory: /
     schedule:
-      interval: "weekly"
+      interval: monthly
+    groups:
+      github-actions:
+        patterns:
+          - "*"