Split out semantic convention into dedicated repo

[jack-berg]

Now that semantic conventions are split out from the specification, we should consider moving the opentelemetry-semconv artifact to its own repository which can be versioned in lockstep with the semantic-conventions. This should help alleviate confusion about which version of semantic conventions are represented in a particular version of opentelemetry-java. Every distinct version of semantic conventions would have its own corresponding version of the opentelemetry-semconv artifact.