Upgrade to .net 6.0 - Githubissues

This is a meta issue regarding upgrading the project to .net 6.0 which is the new LTS version.

[x] Upgrade the projects to support both 3.1 and 6.0
[x] Fix so the builds run for both 3.1 and 6.0
[x] Fix the change from EndpointRoute to HttpContext as the default resource for authorize request #47.

koralium / OPADotNet