update target frameworks to `net461;net5.0;netstandard2.0`
update target frameworks to `net461;net5.0;netstandard2.0`

- use official guidance here: https://docs.microsoft.com/en-us/dotnet/standard/library-guidance/cross-platform-targeting
- reviewed framework lifecycle here: https://devblogs.microsoft.com/dotnet/support-ending-for-the-net-framework-4-4-5-and-4-5-1/

ullmark/hashids.net,ullmark/hashids.net