docs: Fix links to APM Server

[bmorelli25]

https://www.elastic.co/guide/en/elasticsearch/reference/7.6/built-in-users.html#add-built-in-user-apm

This PR fixes links in 7.8 that point to the APM Server reference. These links currently point to the 7.0 version of the documentation. This fix should be backported to 7.7 and 7.6.

The root of this problem is that APM Server does not build a 7.x documentation branch. As a workaround, we often tie 7.x links to other versions of the documentation. These links should be updated at a later date, but sometimes they slip through the cracks 🤷.
To fix this in the Kibana reference, we conditionally change links to point to APM Server's master docs when branch == 7.x:

ifeval::[{branch} == 7.x]
:apm-server-ref:       {apm-server-ref-m}
:apm-overview-ref-v:   {apm-overview-ref-m}
endif::[]

Is this a solution you'd be willing to consider in the ES reference? If so, I can open a PR on master.

[jrodewig]

LGTM. Links go to the right place!

I think implementing attributes similar to the Kibana setup is a great idea. We already have a similar ifeval set up here:

https://github.com/elastic/elasticsearch/blame/master/docs/Versions.asciidoc#L67

Feel free to place the attributes there or in another area of that file.

[elasticmachine]

Pinging @elastic/es-security (:Security/Authentication)

[elasticmachine]

Pinging @elastic/es-docs (>docs)