Update apm package doc

dev docs need to be updated to handle the apm package removal.

Ref https://github.com/elastic/apm-data/blob/main/dev_docs/HOW_TO.md#2-make-changes-in-apm-server

Open dev_docs/HOW_TO.md and read section 2, “Make changes in apm-server,” together with the issue description. Update the development instructions so they accurately handle the apm package removal; the work is done when that section no longer directs contributors through the removed package.

Written by the indexing model from the issue text.