Update C# language version to 14.0 in test project

Changed the LangVersion property from 'preview' to '14.0' in the CommunityToolkit.Mvvm.Roslyn5000.UnitTests.csproj file to explicitly target C# 14.0 for test builds.

Author: Sergio0694

tests/CommunityToolkit.Mvvm.Roslyn5000.UnitTests/CommunityToolkit.Mvvm.Roslyn5000.UnitTests.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <TargetFrameworks>net472;net8.0;net9.0</TargetFrameworks>
-    <LangVersion>preview</LangVersion>
+    <LangVersion>14.0</LangVersion>
     <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings>
     <DefineConstants>$(DefineConstants);ROSLYN_4_12_0_OR_GREATER;ROSLYN_5_0_0_OR_GREATER</DefineConstants>