Update dependabot.yml

- Add update of submodules

.github/dependabot.yml

@@ -1,6 +1,11 @@
 version: 2
 updates:
-  - package-ecosystem: "github-actions"
+- package-ecosystem: "github-actions"
     directory: "/"
     schedule:
       interval: "weekly"
+  - package-ecosystem: "gitsubmodule"
+    directory: "/"  # the .gitmodules file must be placed at the top-level directory of a git working tree
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 15  # Assimp has 15 dependencies