Update CultureMiddleware to use MultiCulture config

Replaced Cultures with MultiCulture in CultureMiddleware.cs.
Constructor now accepts IOptions<MultiCulture> for culture settings.

EnvelopeGenerator.Web/Middleware/CultureMiddleware.cs

@@ -9,9 +9,9 @@ namespace EnvelopeGenerator.Web.Middleware;
 public class CultureMiddleware
 {
     private readonly RequestDelegate _next;
-    private readonly Cultures _cultures;
+    private readonly MultiCulture _cultures;
 
-    public CultureMiddleware(RequestDelegate next, IOptions<Cultures> culturesOpt)
+    public CultureMiddleware(RequestDelegate next, IOptions<MultiCulture> culturesOpt)
     {
         _next = next;
         _cultures = culturesOpt.Value;