Maximize reach wrt FSharp.Core and netcoreapp versions re #6

[bartelink]

I work on a project that supports "FSharp.Core" Version="3.1.2.5" in its net461 variant and makes use of this in samples; relaxing the constraints as this PR does means I can compile those without warnings. I have a similar need/desire to include this in a dotnet new template and having maximal reach is critical in that context.

• reduce .NET Core target to netcoreapp2.1 in order to verify current LTS version works correctly
• add net461 multitargeting to tests - I'm fine with this being removed or changed to net45 if it proves problematic; IME net461 targeting is commonly with default SDK/VS installs
• reduce FSharp.Core version requirement for net45 to Version="3.1.2.5"
• increase FSharp.Core version requirement for netstandard2.0 to Version="4.3.4" (was 4.1.17) - happy to remove this change as TBH I'm not sure how enricosada landed on this
• up package version

[alfonsogarciacaro]

The build script checks wether the version here and in RELEASE_NOTES are out-of-sync to to know when it should push a new release, so you don't need to update this manually :) But I can revert just this line now.

[bartelink]

Ah, makes sense (in https://github.com/jet/jet.jsonnet.converters et al, I use MinVer, which works well wrt inferring a suitable version, but having it tie in with release notes is more important than that).

Feel free to ping if you want me to expand on this (to be clear though: it seems the script and process is already better as-is and if it aligns with processes in sister projects it would be crazy to foist the dependency for something which will hopefully go into maintenance mode very soon!)

Thanks for the speedy merge!