Update to ASP.NET Core 3.1

[robertovaldesperez]

public abstract class BaseValidatingContext : BaseContext

ASP.NET Core 3.1
publicabstract class BaseContext<TOptions> where TOptions : AuthenticationSchemeOptions

[Xacron]

I've been neglecting this project because I'm not certain there are people still interested in deploying their own low-level OAuth server over more complete solutions like IdentityServer. If there's still interest I can dedicate some time to bringing this project back up-to-date.