Fix issue: AbandandMutexException for .net 6 (dotnet#7556)

* Fix sample code for `Mutex` Moved mutex construction to the main method to ensure that the main thread owns the mutex. https://docs.microsoft.com/en-us/dotnet/api/system.threading.mutex?view=net-6.0#examples

Author: TsuyoshiUshio

samples/snippets/csharp/VS_Snippets_CLR_System/system.Threading.Mutex 1Arg Ctor Example/CS/class1.cs

@@ -4,13 +4,14 @@
 
 class Test
 {
- // Create a new Mutex. The creating thread owns the Mutex.
- private static Mutex mut = new Mutex(true);
+ private static Mutex mut;
  private const int numIterations = 1;
  private const int numThreads = 3;
 
  static void Main()
  {
+ // Create a new Mutex. The creating thread owns the Mutex.
+ mut = new Mutex(true);
  // Create the threads that will use the protected resource.
  for(int i = 0; i < numThreads; i++)
  {

samples/snippets/csharp/VS_Snippets_CLR_System/system.Threading.Mutex 1Arg Ctor Example/CS/class1.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+ <PropertyGroup>
+ <OutputType>Exe</OutputType>
+ <TargetFramework>net6.0</TargetFramework>
+ </PropertyGroup>
+
+</Project>