Do not call UseContentRoot() on the web host builder

This call has nothing to do with static web content and should not have ever been called with the web content path: https://docs.microsoft.com/en-us/aspnet/core/fundamentals/?view=aspnetcore-3.1&tabs=windows#content-root
Since this call is removed, the ContentRoot property is also removed from ApplicationHost as it is no longer needed.
Finally, move validation of the static content path to the DashboardService where it is actually used.

MediaBrowser.WebDashboard/Api/DashboardService.cs

@@ -136,6 +136,17 @@ namespace MediaBrowser.WebDashboard.Api
             _resultFactory = resultFactory;
             _resourceFileManager = resourceFileManager;
             _appConfig = appConfig;
+
+            // Validate web content path
+            string webContentPath = DashboardUIPath;
+            bool webContentPathValid = appConfig.NoWebContent() || (Directory.Exists(webContentPath) && Directory.GetFiles(webContentPath).Any());
+            if (!webContentPathValid)
+            {
+                throw new InvalidOperationException(
+                    "The server is expected to host web content, but the provided content directory is either " +
+                    $"invalid or empty: {webContentPath}. If you do not want to host web content with the server, " +
+                    $"you may set the '{Controller.Extensions.ConfigurationExtensions.NoWebContentKey}' flag.");
+            }
         }
 
         /// <summary>