Upgraded all project from .NET 4.6 to .NET 4.6.2 that also .NET framework applications can use latest version of Kestrel
* Updated Kestrel to 2.2.0 (for Opc.Ua.Core)
* Simplified csproj (removed ItemGroup Condition="'$(TargetFramework)' == 'netstandard1.3'") because currently we don't build netstandard1.x anymore
* Removed Kestrel 1.1.3 code from Stack/Opc.Ua.Core/Stack/Https/HttpsListener.cs
* add conditions to Stack/Opc.Ua.Core/Stack/Https/HttpsTransportChannel.cs
* [HttpClientHandler.ServerCertificateCustomValidationCallback](https://docs.microsoft.com/en-us/dotnet/api/system.net.http.httpclienthandler.servercertificatecustomvalidationcallback?view=netcore-3.1&viewFallbackFrom=netframework-4.6.2) should not be available in .NET 4.6.X
* Updated travis.ci to use latest mono version (original version causes build errors)
* Added nuget reference .NET Standard Library to mono projects to be compatible with latest Kestrel version
Own Travis.ci and Azure Pipeline builds run sucessfully.
2. Removed client cert validation from HTTPS client and server as this is not supported on all platforms, e.g. .Net4.6.
3. Reverted back to XmlDocument from XDocument as this is now supported via a NuGet package on UWP and XDocument is not supported on .Net4.6.
4. Baseline .Net4.6 samples and assoc. controls project check-in, not working yet.
5. Moved UWP client controls from SDK to Samples directly.
Alexander Köpke