Remove temporary APIs before .NET 8 ships

[SteveSandersonMS]

When the Razor compiler is updated, we need to remove:

• Everything in src\Components\Web\src\RenderMode\TemporaryRenderModeAttributes.cs, since they will be superseded by types used with the @rendermode directive:
  • RenderModeServerAttribute
  • RenderModeWebAssemblyAttribute
  • RenderModeAutoAttribute
• RenderTreeBuilder.AddComponentParameter(int, string, IComponentRenderMode) (since it will be superseded by AddComponentRenderMode). Can then also remove _pendingComponentCallSiteRenderMode.
• Logic inside RenderTreeBuilder.AddAttribute that specifically checks for the attribute name "@formname", since that will be superseded by AddNamedEvent. Can then also remove _pendingNamedSubmitEvent.
• Update the project template, obviously, or it will no longer work
  • Add a using static so we don't need the RenderMode. prefix

[SteveSandersonMS]

I'm moving this to RC2 just to make sure it doesn't get forgotten. I think it actually has to be done for GA, not RC2, but I'm not sure where that shows up in the project boards. We can move it to GA when there's a correct way to do that.