Add .NET6 target for Android

[frederikprijck]

Changes

Add support to run our Android SDK on both Xamarin and .NET Android applications. To compile for both platforms, we need to use Xamarin.Legacy.Sdk .

Testing

Please describe how this can be tested by reviewers. Be specific about anything not tested and reasons why. If this library has unit and/or integration testing, tests should be added for new functionality and existing tests should complete without errors.

• [x] This change adds unit test coverage
• [x] This change adds integration test coverage
• [x] This change has been tested on the latest version of the platform/language or why not

Checklist

• [x] I have read the Auth0 general contribution guidelines
• [x] I have read the Auth0 Code of Conduct
• [x] All existing and new tests complete without errors
• [x] All code guidelines in the CONTRIBUTING documentation have been run/followed