Remove schema1 API support and default to schema2 manifests

Related to #213 (closed)

This MR removes the handler code for schema1 manifests as well as related configuration options and documentation.

Additionally, API tests which rely on schema1 manifests were updated to schema2 and tests which cover schema1 manifests were removed.

There are still tests throughout the registry which rely on schema1 #265 so some internal service code which handles schema 1 manifests remains, as well as type definitions for schema1 manifests.