Create semconv-rpc-approvers group

With https://github.com/open-telemetry/community/blob/main/projects/rpc-semconv.md up and running, we need approvers group to triage and review RPC-related PRs in semantic-conventions repo. 

Initial members: @trask @lmolkova 

Similarly to other semconv-*-approvers group, this one will probably need to be a collaborator on https://github.com/open-telemetry/semantic-conventions

