codegen: remove "V2" suffix from endpoint resolution codegen

We implemented codegen for `@endpointRuleSet` to generate `EndpointResolverV2` because `EndpointResolver` as a name was already taken downstream by the go v2 SDK's existing endpoint resolver.

We need to separate the concerns of the SDK from smithy-go in general, the smithy-go codegen should be rebranded to generate as `EndpointResolver` and the SDK will have to customize around it somehow.

Start by tracing the @endpointRuleSet codegen entry point and the generated EndpointResolverV2 output, then inspect how the Go v2 SDK currently uses the existing EndpointResolver name. Done means smithy-go generates EndpointResolver while the SDK can customize around that name without the current collision.

Written by the indexing model from the issue text.