title: UseSpaStaticFiles() not found after migrating to dotnet Core 3.1
body: <p>After upgrading my <code>.NetCore 2.2</code> application to <code>Net Core 3.1</code>. I am now facing issue in the <code>startup.cs</code> file related to a <code>UseSpaStaticFiles()</code> method.</p>
  
  <pre><code>    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
      {
          app
              .UseCors("default")
              .UseStaticFiles()
              .UseSpaStaticFiles(); // this line is giving me error.
         .....
      }
  </code></pre>
  
  <p>The error says : </p>
  
  <blockquote>
    <p>Error CS1061  'IApplicationBuilder' does not contain a definition for
    'UseSpaStaticFiles' and no accessible extension method
    'UseSpaStaticFiles' accepting a first argument of type
    'IApplicationBuilder' could be found (are you missing a using
    directive or an assembly reference?)</p>
  </blockquote>
  
  <p>To resolve the issue I have tried installing <code>Microsoft.AspNetCore.SpaServices</code> package from <a href="https://www.nuget.org/packages/Microsoft.AspNetCore.SpaServices/3.1.0?_src=template" rel="noreferrer">Nuget</a> and that also didn't help.</p>