Upgrade Elastic Docs should include APM

👋🏼 [Upgrade Elastic on-prem](https://www.elastic.co/guide/en/elastic-stack/8.1/upgrading-elastic-stack-on-prem.html) Docs don't currently cover where in the sequence you upgrade [APM Server](https://www.elastic.co/guide/en/apm/index.html) or >7.15 Integations Server. Do we consider this unapplicable or asynchronously upgraded?