Blazor doesn't always dispose the circuit's DI scope

[SteveSandersonMS]

From #11791

• In CircuitHost.DisposeAsync, use try/finally to guarantee _scope.Dispose still happens even if any of OnConnectionDownAsync/OnCircuitDownAsync/Renderer.Dispose throws
• In CircuitPrerenderer.PrerenderComponentAsync, use try/finally to call CleanupCircuitState if prerendering throws