Update FSharp.Core dependency

[Thorium]

This updates FSharp core to latest stable, but nothing else.

[Thorium]

This is because some existing unit-tests seems to do add reference to FSharp.Core without declaring a version, so it downloads a latest, which used to be F# 6 but is now F# 8, so then the unit-tests raise something like:

"Paket resolved a FSharp.Core with version 8.x.x, but fake runs with a version of '6.x.x'. This is not supported. Please either lock the version via 'nuget FSharp.Core ' or upgrade fake. Read #2001 for details."

FSharp.Core dependency version is independent of any target-frameworks, so this is not touching that, target is still .NET6.