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

[trentm]

I noticed in elastic/apm-agent-nodejs#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).