Server-side Blazor Project template contains incorrect comment

[SteveSandersonMS]

In _Host.cshtml:

    <app>
        @* Remove the following line of code to disable prerendering *@
        @(await Html.RenderComponentAsync<App>(RenderMode.ServerPrerendered))
    </app>

Since #13147, the comment is no longer correct - following that advice will now just break the app.

The API is now self-explanatory enough that I suggest we just remove the comment. People who want to disable prerendering will be able to guess that they do so by changing to a different RenderMode. It's a great API :)

Also it should be trivial to eliminate the <app> element too. We just have to change the CSS so that rules targetting app target body instead (but verify this).