Remove apmpackage

[axw]

Motivation/summary

Remove apmpackage from the apm-server repo, as we are moving the integration package to elastic/integrations.

A couple of things have led to this:

• We are currently making minimal changes to the integration package.
• We are going to switch to an Elasticsearch plugin (apm-data) for installing index templates and ingest pipelines, and use dynamic mapping more heavily to make the apm-server code and templates more loosely coupled.

The integration package will still exist, but will be "input-only", meaning that it will not define data streams; it will only be used to define the apm-server configuration/policy vars. To align with other packages, we will move the integration package to the elastic/integrations repo. We're doing it now, rather than waiting for the switchover to the ES plugin, to make progress on phasing out some legacy CI infrastructure.

What does this mean for development?

• We won't be able to atomically make changes to the data stream templates and pipelines along with code. This will be the case when we are switched over to the plugin, we're just bringing that forward.
• System tests, and automated benchmarks and Elastic Cloud deployments will use the integration package bundled with Kibana, rather than building and installing the package by upload. The Kibana config has been updated to automatically install the bundled integration package.

Requires elastic/integrations#8881

Checklist

- [ ] Update CHANGELOG.asciidoc
- [ ] Update package changelog.yml (only if changes to apmpackage have been made)
- [ ] Documentation has been updated

How to test these changes

N/A

Related issues

#11529
#11528

[mergify]

This pull request does not have a backport label. Could you fix it @axw? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.

NOTE: backport-skip has been added to this pull request.

[mergify]

This pull request is now in conflicts. Could you fix it @axw? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b mv-apmpackage upstream/mv-apmpackage
git merge upstream/main
git push upstream mv-apmpackage