stop setting `span.id`, set `transaction.id` for log correlation

I noticed in https://github.com/elastic/apm-agent-nodejs/pull/3399 discussion that, while the ecs-logging-nodejs libs are currently setting `span.id` or `transaction.id` for log-correlation, the ecs-logging spec (https://github.com/elastic/ecs-logging/blob/main/spec/spec.json) only mentions setting `transaction.id`. So it appears these libs have this wrong.

TODO:

- [ ] Confirm whether log-correlation works for log records that set `trace.id` and `span.id`. I.e. are these libs breaking log correlation for some log records?
- [ ] Change to conform to the spec (and what the Java APM agent is doing).