Add a net8.0 target

[m-redding]

Confirm this is a feature request for the .NET library and not the underlying OpenAI API

• This is a feature request for the .NET library

Describe the feature or improvement you are requesting

It would be great if OpenAI could add a net8.0 target, in order to unblock adding a net8.0 target to Azure.AI.OpenAI. There are more details in this issue: Azure/azure-sdk-for-net#46891

Alternatively, resolving #259 would also unblock adding this target to Azure.AI.OpenAI

Additional context

No response