Update target frameworks

[Joy-less]

This pull request removes unsupported target frameworks (.NET Core, .NET 6.0) and simplifies conditional compilation so it supports NETSTANDARD (.NET Standard 2.0) and NET (.NET 8.0 and .NET 9.0).

[bitfaster]

Wow, thanks!

I had started to look at this here #638, and planned to go back and effectively make your PR (so I would go in stages update fwks changes, then take each feature as a follow PR).

I think the gates will fail on your change, I just started a run - my other PR has fixes for that stuff in the build yaml files.

I will take a proper look at this on the weekend.

[bitfaster]

I updated the build scripts so that your changes can compile: #670.

[Joy-less]

I updated the build scripts so that your changes can compile: #670.

I merged the changes.

[bitfaster]

Build is failing because deleting framework packages is not backwards compatible (i.e. a breaking change), so this would have to be a major release:

Error: C:\Program Files\dotnet\sdk\9.0.200\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.ApiCompat.ValidatePackage.targets(39,5): error CP0002: Member 'System.Threading.Tasks.ValueTask<V> BitFaster.Caching.IAsyncCache<K, V>.GetOrAddAsync<TArg>(K, System.Func<K, TArg, System.Threading.Tasks.Task<V>>, TArg)' exists on [Baseline] lib/net6.0/BitFaster.Caching.dll but not on lib/netstandard2.0/BitFaster.Caching.dll [D:\a\BitFaster.Caching\BitFaster.Caching\BitFaster.Caching\BitFaster.Caching.csproj]

[Joy-less]

Build is failing because deleting framework packages is not backwards compatible (i.e. a breaking change), so this would have to be a major release

Is there going to be a major release soon? What's the plan?