Upgrade C# Protobuf projects to .NET 8.0.

This change updates the target frameworks for various C# projects within Protobuf to .NET 8.0, while also maintaining netstandard2.0 for the core library. Support for older frameworks like net45, net50, and netstandard1.1 has been removed.

This updates our project to match our support policy in https://github.com/google/oss-policies-info/blob/main/foundational-dotnet-support-matrix.md which actually dropped support for net45, net50, etc long ago.

PiperOrigin-RevId: 919111969