Auto update dependencies for web_ui. (flutter/engine#45754)

This is to autofix problems like: https://github.com/flutter/engine/security/dependabot

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

engine/src/flutter/.github/dependabot.yml

@@ -15,3 +15,9 @@ updates:
       # ignore patch versions, just rely on minor in order to update fewer times
       - dependency-name: "github/codeql-action"
         update-types: ["version-update:semver-minor"]
+  - package-ecosystem: "pub"
+    directory: "/lib/web_ui/pubspec.yaml"
+    schedule:
+      interval: "daily"
+    labels:
+      - "autosubmit"