refactor!: revert "feat: use superclass of VaadinServlet/Request (#9699)"

[pleku]

This reverts commit c581668, except
intentionally leaves out the protected API introduced to StaticFileServer to get a static
resource, so it may be overridden to use something else than VaadinServletService.

[pleku]

I'll fix the commit message, the PushRequestHandler + PushHandler parts were also reverted because this change also requires that VaadinServletRequest would accept VaadinService since for some reason (at least for detecting the session), PushHandler creates a VaadinServletRequest from the received AtmosphereRequest. The code (existing before contribution) is a bit off here - only a VaadinRequest would actually be needed since the PushHandler uses only VaadinService which only needs VaadinRequest, not VaadinServletRequest. But since there is no way to just make an instance of VaadinRequest, the PushHandler code is tied to VaadinServletRequest instead.

This part is reverted for now. Making PushRequestHandler and PushHandler not rely on VaadinServletService is doable separately later on, but not a priority. It should be enough to make some kind of wrapper for VaadinRequest EDIT: actually at the moment the is no case where Atmosphere support would be used without servlet

[vaadin-bot]

SonarQube analysis reported 6 issues

Note: The following issues were found on lines that were not modified in the pull request. Because these issues can't be reported as line comments, they are summarized here:

1. VaadinService.java#L413: Rename "instantiator" which hides the field declared at line 192.
2. VaadinService.java#L354: Replace this lambda with a method reference.
3. VaadinService.java#L416: Remove this use of "init"; it is deprecated.
4. VaadinService.java#L430: Remove this use of "init"; it is deprecated.
5. VaadinServlet.java#L196: Remove the declaration of thrown exception 'javax.servlet.ServletException', as it cannot be thrown from method's body.
6. VaadinServlet.java#L545: Replace this lambda with a method reference.