Bump F# to 8.0 to compile with .NET 11 SDK

This fixes the following compile error:
```
FSC : error FS3880: Language version '6.0' is out of support. The last .NET SDK supporting it is available at https://dotnet.microsoft.com/en-us/download/dotnet/10.0
```

Author: jnyrup

Tests/FSharp.Specs/FSharp.Specs.fsproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <TargetFramework>net6.0</TargetFramework>
-    <LangVersion>6.0</LangVersion>
+    <LangVersion>8.0</LangVersion>
     <DisableImplicitFSharpCoreReference>true</DisableImplicitFSharpCoreReference>
   </PropertyGroup>