[maintenance] add gradle deps to dependabot tracking (#8924)

Fixes: #8922.

---

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<details>
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  the [Flutter organization contributor guide]([https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md)
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Author: pq

.github/dependabot.yml

@@ -11,3 +11,10 @@ updates:
     ignore:
       - dependency-name: "*"
         update-types: ["version-update:semver-patch"]
+
+  - package-ecosystem: "gradle"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    labels:
+      - "autosubmit"