Update package:web_benchmarks dep. (#8530)

This change is in preparation of the removal of `--web-renderer=html`.

* Related: flutter/packages#8103 (tooling change to remove the parameter from the benchmark runner)
* Related: flutter/flutter#145954 (general thread)

## Tests

In this case, CI should continue normally. The change is just adopting a new major version of `package:web_benchmarks` that removed some public API (that wasn't used in devtools).

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packages/devtools_app/pubspec.yaml

@@ -76,7 +76,7 @@ dev_dependencies:
   mockito: ^5.4.1
   stager: ^1.0.1
   test: ^1.21.0
-  web_benchmarks: ^3.1.0
+  web_benchmarks: ^4.0.0
   webkit_inspection_protocol: ">=0.5.0 <2.0.0"
 
 flutter: