Akka.FSharp: need to pin FSharp.Core package version to 6.0

[Aaronontheweb]

Version Information
Version of Akka.NET? v1.4.47 and later
Which Akka.NET Modules? Akka.FSharp, Akka.Persistence.FSharp

Describe the bug

A F# Akka.NET user reported in Discord that Akka.NET triggers a NuGet package downgrade warning when installing into a .NET 6 solution in F#:

  Akka1.5.3Test.fsproj: [NU1605] Detected package downgrade: FSharp.Core from 7.0.0 to 6.0.5. Reference the package directly from the project to select a different version. 
 Akka1.5.3Test -> Akka.FSharp 1.5.4 -> FSharp.Core (>= 7.0.0) 
 Akka1.5.3Test -> FSharp.Core (>= 6.0.5)

To Reproduce

Full repo from @hkirk here https://github.com/hkirk/Akka1.5.3Test

Expected behavior

Akka.FSharp should install normally into a .NET 6 application with no NuGet package downgrade warnings.

Screenshots
If applicable, add screenshots to help explain your problem.

Environment

.NET 6 and below - anything below .NET 7.

Additional context

Obviously this doesn't happen - the issue occurred in Akka.NET v1.4.47 when we removed explicit references to Akka.FSharp here #6641 and bumped to 7.x here #6402.

Both of these PRs should be reverted.

[Aaronontheweb]

Both of these PRs should be reverted.

And FSharp.Core should be set to 6.x

[Aaronontheweb]

https://github.com/dotnet/fsharp/blob/main/docs/fsharp-core-notes.md#package-authors-should-pin-their-fsharpcore-reference

[Aaronontheweb]

We should also, and this can be done in a separate PR, tell Dependabot not to upgrade FSharp.Core.

[hkirk]

Hej @Aaronontheweb I still get this error on version 1.5.4-1.5.7.

I've updated the repository with newest version of Akka and Akka.FSharp

[hkirk]

I have a potential fix (work around) here: https://github.com/hkirk/Akka1.5.3Test/tree/work_around
But not sure this is the correct way.