[APM] Remove the watcher integration and document the watcher error reporting

[nehaduggal]

Documentation issue #70448

We currently offer two ways of setting up error rate alerts via watcher and via the new alerting UI in Kibana. In order to keep the alerting simple, we should get rid of the older watcher error rate integration and remove that dropdown from the APM App. We should document this removal in the UI and provide the recipe to create the watcher error in the documentation.

@formgeist @sqren

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[formgeist]

We should document this removal in the UI and provide the recipe to create the watcher error in the documentation.

IMO it would be sufficient to announce it in the release notes and add it in the documentation, but perhaps that's what you mean?

[formgeist]

I opened #70448 to handle the documentation side. We can move this into implementation in order to remove the Integrations option entirely.