Set Min OS version for ALPN tests at Win 8.1.

test/Kestrel.FunctionalTests/Http2/TlsTests.cs

@@ -25,6 +25,8 @@ using Xunit;
 namespace Microsoft.AspNetCore.Server.Kestrel.FunctionalTests.Http2
 {
     [OSSkipCondition(OperatingSystems.MacOSX, SkipReason = "Missing SslStream ALPN support: https://github.com/dotnet/corefx/issues/30492")]
+    [MinimumOSVersion(OperatingSystems.Windows, WindowsVersions.Win81,
+        SkipReason = "Missing Windows ALPN support: https://en.wikipedia.org/wiki/Application-Layer_Protocol_Negotiation#Support")]
     public class TlsTests : LoggedTest
     {
         private static X509Certificate2 _x509Certificate2 = TestResources.GetTestCertificate();