event.id -> http.request.id in `formatHttpRequest`

[trentm]

ECS 1.9.0 formalizes http.request.id: https://github.com/elastic/ecs/releases/tag/v1.9.0

Currently formatHttpRequest will map req.id to event.id. That isn't documented or in the tests. Judgement call if that mandates a breaking change to remove.