Update the default fsharp language version to 5.0

[KevinRansom]

Fixes #10144

mea culpa ... throw big sticks at me over this, I completely missed it.

[cartermp]

LanguageFeature.OverloadsForCustomOperations should also be lifted to 50. It works great. Otherwise, this is good

[KevinRansom]

@cartermp , I will submit a seperate PR for overloads for custom operators. I know it seems silly but revert and bisect for failures is easier. And I expect that will require some test updating too.

[dsyme]

All of these are still listed as preview is that right??

            LanguageFeature.FromEndSlicing, previewVersion
            LanguageFeature.OpenTypeDeclaration, previewVersion
            LanguageFeature.PackageManagement, previewVersion
            LanguageFeature.WitnessPassing, previewVersion
            LanguageFeature.InterfacesWithMultipleGenericInstantiation, previewVersion
            LanguageFeature.NameOf, previewVersion
            LanguageFeature.StringInterpolation, previewVersion

[cartermp]

No, only in main. I have no idea why main isn't showing this though.

https://github.com/dotnet/fsharp/blob/release/dev16.8/src/fsharp/LanguageFeatures.fs#L63

[KevinRansom]

Once dev16.8 branch has rtm'd we will integrate the changes back into master. We make version changes like this only in the release branches like dev16.8. Once the release branches have rtmd they are integrated back into master. It's just the process we use and it works out fine.