Update prerelease label to preview2
Update prerelease label to preview2

release/uwp6.0 will use preview1

hongdai/wcf,ericstj/wcf,StephenBonikowsky/wcf,KKhurin/wcf,dotnet/wcf,hongdai/wcf,KKhurin/wcf,ElJerry/wcf,imcarolwang/wcf,ericstj/wcf,imcarolwang/wcf,zhenlan/wcf,dotnet/wcf,ElJerry/wcf,imcarolwang/wcf,zhenlan/wcf,StephenBonikowsky/wcf,mconnew/wcf,mconnew/wcf,mconnew/wcf,dotnet/wcf