Target .Net standard 2.0

[ImrePyhvel]

.Net Standard 2.0 can be consumed by both .net FW and .net core/.net5 projects and also works when Task library is being loaded by dotnet tool "small" msbuild.

[ImrePyhvel]

Upgraded Task asssembly to .Net standard 2.0.

Test project upgraded to .net5.

Nortal.Utilities.TextTemplating 0.10.0 is internally used from .Net FW version, but seems to work ok.