Run extension method to register the simplest middleware

Startup.cs

public class Startup
{
    // This method gets called by the runtime. Use this method to add services to the container.
    // For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940
    public void ConfigureServices(IServiceCollection services)
    {
    }

    // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        // Run method, anonymous inline middleware, 
        // Terminal middleware as it ends the app pipeline.
        // NOTE: Run method does not have ability to call next middleware.
        app.Run(async context =>
        {
            await context.Response.WriteAsync("My First .NET 5 Web App");
        });
    }
}