Suboptimal version of System.Threading.Tasks.Extensions dependency for target net461

[pr0Igor]

Hi,
Framework.Core.csproj package reference System.Threading.Tasks.Extensions version 4.5.1 set for target net461 looks to be of the wrong version.

Version 4.5.1 does not have assemblies for the net461 target, .NET Standard will be used instead.

Build output for projects targeting net461, with added Theraot.Core, will contain the .NET Standard reference assemblies.
(Build couldn't resolve .net461, the best(only?) match was to use net Standard?)

Bumping the version of System.Threading.Tasks.Extensions 4.5.1 > 4.5.4 will resolve the situation.
(The problem can also be quick fixed by referencing the package System.Threading.Tasks.Extensions version 4.5.4
in the same project that references the Theraot.Core).

Kind regards and thank you for the library.