Updating new test to require MSBuild version

Author: sfoslund

src/Tests/Microsoft.NET.Build.Tests/GivenThatWeWantToProduceReferenceAssembly.cs

@@ -18,7 +18,7 @@ public class GivenThatWeWantToProduceReferenceAssembly : SdkTest
         public GivenThatWeWantToProduceReferenceAssembly(ITestOutputHelper log) : base(log)
         {}
 
-        [Theory]
+        [WindowsOnlyRequiresMSBuildVersionFact("16.8.0")]
         [InlineData("netcoreapp3.1", false)]
         [InlineData("net5.0", true)]
         public void It_produces_ref_assembly_for_appropriate_frameworks(string targetFramework, bool expectedExists)