Upgraded project to .NET 8.0 because .NET 6.0 is out of support

Source/AsyncProgressBar/AsyncProgressBar.cs

@@ -21,6 +21,10 @@ using System;
 using System.Threading;
 using System.Windows.Forms;
 
+#if NET6_0_OR_GREATER
+using System.Runtime.Versioning;
+#endif
+
 namespace Nuclex.Windows.Forms {
 
   /// <summary>Progress bar with optimized multi-threading behavior</summary>
@@ -37,6 +41,9 @@ namespace Nuclex.Windows.Forms {
   ///     worker thread, increasing its performance.
   ///   </para>
   /// </remarks>
+#if NET6_0_OR_GREATER
+  [SupportedOSPlatform("windows")]
+#endif
   public partial class AsyncProgressBar : ProgressBar {
 
     /// <summary>Initializes a new asynchronous progress bar</summary>