Drop .NET Standard 2.0 Target

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Task description

See #1040 for context. Given the performance penalty for using the netstandard2.0 target, and lack of currently-supported .NET implementations that support it but are not otherwise covered by our other targets, we should drop the netstandard2.0 target. This can be done independently of dropping netstandard2.1, which might remain around for Unity support.

Any libraries that depend on Lucene.NET via netstandard2.0 are encouraged to target net462 and net8.0 (at this time) for better performance. .NET Standard 2.0 lacks performance-optimized APIs and thus causes a performance penalty compared to other targets if you use Lucene.NET in this way.