Remove QtWebEngine hard dependency from OAuth2 doc snippets application

Recent Qt 6.9 have added a documentation snippet application to QtNetworkAuth.
One of the new features it provides documentation snippets for, is QtWebEngine usage.

Currently the snippet application unconditionally links against QtWebEngine, meaning that it must also be present when building the snippet application. This is not a general problem for Qt library users, or Qt integration, but it is not optimal for Qt developers wanting to update the snippet application in the future (to compile it, they need to also have QtWebEngine).

This task is about removing this hard dependency so that the application can be compiled also without QtWebEngine.