Update Paket.Core to target net8.0

[chinwobble]

There are still large performance improvements to be made in Requirements.fs as there of intermediate string heap allocations.

Some of these performance improvements only be made with https://learn.microsoft.com/en-us/dotnet/api/system.string.create?view=net-10.0

@forki is it possible we can update Paket.Core to netstandard 2.1 or net8.0?
Is there still a use case to support net461.
https://dotnet.microsoft.com/en-us/platform/support/policy/dotnet-framework

[Numpsy]

Is there still a use case to support net461.

On a related note, the latest versions of the NuGet client libraries (which Paket.Core depends on) now only support Framework 4.7.2+, so using 4.6.1 could be an issue if those need to be updated.