Remove "experimental" from contour-authserver

[youngnick]

This issue covers work to harden contour-authserver so that people could conceivably use it as a production-quality translation layer between Envoy ext_authz and other forms of RBAC. The idea here is that we can say confidently that Contour can support OIDC as close to out-of-the-box as possible.

Note that the right way to do this may be to send contour-authserver to the Envoy project, or add support into OIDC providers for Envoy's ext_authz directly.