Use Roslyn API for C# language-version check

Use the Roslyn 5+ API when available to determine the C# language version. Add a conditional compilation branch that calls Compilation.HasLanguageVersionAtLeastEqualTo(LanguageVersion.CSharp14) when ROSLYN_5_0_0_OR_GREATER is defined, and fall back to the previous IsLanguageVersionPreview() check otherwise. This avoids generating invalid code on older Roslyn builds while correctly detecting C#14+ in newer toolchains.

Author: Sergio0694

src/CommunityToolkit.Mvvm.CodeFixers/UsePartialPropertyForObservablePropertyCodeFixer.cs

@@ -82,7 +82,11 @@ public override async Task RegisterCodeFixesAsync(CodeFixContext context)
         SemanticModel semanticModel = (await context.Document.GetSemanticModelAsync(context.CancellationToken).ConfigureAwait(false))!;
 
         // If the language is not preview, we cannot apply this code fix (as it would generate invalid C# code)
+#if ROSLYN_5_0_0_OR_GREATER
+        if (!semanticModel.Compilation.HasLanguageVersionAtLeastEqualTo(LanguageVersion.CSharp14))
+#else
         if (!semanticModel.Compilation.IsLanguageVersionPreview())
+#endif
         {
             return;
         }