OAuth2IntrospectionHandler uses ISystemClock that is obsolete in .NET 8

[nuttytree]

In .NET 8 Microsoft made the ISystemClock interface obsolete. In my code I have a need to extend the OAuth2IntrospectionHandler but now that I am working to get ready for .NET 8 I get compile errors because of the obsolete ISystemClock. In general a NuGet should be targeting all currently supported versions of .NET both because it gives consumers of the NuGet confidence that the code has been evaluated and tested against the newer versions of .NET and so that changes like this can be accounted for based on the target version of the NuGet build.

[askids]

I have submitted a PR #191 to add .Net 8.0 support.