Add modifies/dependencies label to dependabot (#36206)

`actions/labeler` can not detect dependency updates in actions because
it works on file level, so we need to let dependabot set this label.

.github/dependabot.yml

@@ -2,7 +2,7 @@ version: 2
 
 updates:
   - package-ecosystem: github-actions
-    labels: []
+    labels: [modifies/dependencies]
     directory: /
     schedule:
       interval: daily