Cleanup vcpkg manifest for pubsub + OTel

Our `opentelemetry` feature should depend on `monitoring` in our vcpkg port. We should not have to manually depend on it in this example's manifest file.

https://github.com/GoogleCloudPlatform/cpp-samples/blob/f214349286e78b1fa5466efa82196e2bc76fd0d1/pubsub-open-telemetry/vcpkg.json#L12

We can remove the workaround when we
- update our vcpkg port
- update the vcpkg version used in our CI

从链接所在行的 pubsub-open-telemetry/vcpkg.json 开始，然后找到 vcpkg port 以及选择其 vcpkg 版本的 CI 配置。完成的标准是 manifest 不再声明 monitoring workaround，opentelemetry feature 提供该依赖项，并且 CI 使用更新后的 vcpkg 版本。

由索引模型根据 Issue 内容生成。