fix: update sitemap generation to use XmlWriter for improved XML handling

- Changed the encoding in the XML declaration from "UTF-8" to "utf-8" for consistency.
- Replaced StringWriter with XmlWriter to enhance XML document writing and ensure proper encoding settings.
- Updated the return statement to convert the MemoryStream to a UTF-8 string, improving the efficiency of the sitemap generation process.

These changes enhance the reliability and correctness of the sitemap generation, ensuring better compliance with XML standards.

src/FrontOffice.Main/Utilities/Seo/SitemapGenerator.cs

@@ -1,4 +1,5 @@
 using System.Text;
+using System.Xml;
 using System.Xml.Linq;
 using Microsoft.Extensions.Options;
 
@@ -71,16 +72,23 @@ public class SitemapGenerator
         }
 
         var document = new XDocument(
-            new XDeclaration("1.0", "UTF-8", null),
+            new XDeclaration("1.0", "utf-8", null),
             new XElement(ns + "urlset", urls));
 
-        var builder = new StringBuilder();
-        using (var writer = new StringWriter(builder))
+        var settings = new XmlWriterSettings
+        {
+            Encoding = new UTF8Encoding(encoderShouldEmitUTF8Identifier: false),
+            Indent = true,
+            OmitXmlDeclaration = false
+        };
+
+        using var stream = new MemoryStream();
+        using (var writer = XmlWriter.Create(stream, settings))
         {
             document.Save(writer);
         }
 
-        return builder.ToString();
+        return Encoding.UTF8.GetString(stream.ToArray());
     }
 
     private static XElement CreateUrlElement(