Fix implicit dependency on FSharp.Core

[dawedawe]

fixes #176

[SchlenkR]

Just to clarify: Will this change (having FsHttp reference F# Core 8) be working for FsHttp users targeting combinations of dotnet SDK / F# with versions lower than 8?

[dawedawe]

Just to clarify: Will this change (having FsHttp reference F# Core 8) be working for FsHttp users targeting combinations of dotnet SDK / F# with versions lower than 8?

The combinations should still work as before.
I checked with a project having TargetFramework = net6.0 and a local nuget source.
We are already doing this for FsHttp.FSharpData.
The explicit dependencies of FsHttp.FSharpData can also be seen here
The FsHttp package shows the missing explicit dependency here, that should be fixed with 14.4.1.

[SchlenkR]

Thank you so much for taking your time @dawedawe for trying out and sharing, and the PR. Great stuff!