Added missing content-type header to BootFailedMiddleware response

When Umbraco is configured with X-Content-Type-Options nosniff as is recommended (there's even a health check for it), any boot failure would simply show the html source code in the browser instead of properly rendering the html, as the content-type wasn't being set.

(cherry picked from commit d4c921264d)

src/Umbraco.Web.Common/Middleware/BootFailedMiddleware.cs

@@ -1,3 +1,4 @@
+using System.Net.Mime;
 using System.Text;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Http;
@@ -51,6 +52,7 @@ public class BootFailedMiddleware : IMiddleware
                 // Print a nice error page
                 context.Response.Clear();
                 context.Response.StatusCode = 500;
+                context.Response.ContentType = MediaTypeNames.Text.Html;
 
                 IFileInfo? fileInfo = GetBootErrorFileInfo();
                 if (fileInfo is not null)