This commit:
- Updates the .NET versions which are being targetted to:
* .NET 4.6.2 (.NET 4.5 has been out of support for a very long time)
* net6.0 and net8.0, net5.0 and netstandard have been out of support for a very long time
- Updates the various NuGet packages which are referenced to the latest version
While we're at it, clean up the conditional compliation constants and dependencies:
- Define `SPAN_NATIVE` on all platforms _except_ `net45` and `netstandard2.0` (so it's defined automatically on new .NET versions)
- Only add `System.Memory` dependency on `net45` and `netstandard2.0`.
- Fix `NATIVE_SPAN` typo
- Remove obsolete `NETCORE` preprocessor variable.