[go-server] Return Router interface from controller constructor.

Created by: Jesse0Michael

Update the go server api controller template to return the Router interface instead of the api specific router. The Router interface type has the Routes function, which is what the generated NewRouter function needs. (not the api specific interface).

The change added in https://github.com/OpenAPITools/openapi-generator/pull/4038 would cause the following error since it was working with the wrong interface type

./main.go:38:36: cannot use PetApiController (type petstoreserver.PetApiRouter) as type petstoreserver.Router in argument to petstoreserver.NewRouter:
        petstoreserver.PetApiRouter does not implement petstoreserver.Router (missing Routes method)

@antihax @bvwells @grokify @kemokemo @bkabrda

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