[DOCS] Supporting Upgrades from 6.x to 7.x

This issue is meant to give a high level overview of all the work needed to update the documentation for users upgrading from 6.x to 7.x.

Installation and Upgrade Guide (https://www.elastic.co/guide/en/elastic-stack/7.0/index.html):

- [x] Update upgrade steps. See https://github.com/elastic/stack-docs/pull/242
- [x] Verify list of "notable" breaking changes.
- [ ] Address feedback in https://github.com/elastic/stack-docs/issues/226
- [x] Add a topic that discusses the benefits of upgrading. See https://github.com/elastic/stack-docs/pull/287
- [ ] Provide information about how to fix a “broken” upgrade if you upgrade from a pre-6.7 version and do not use the Migration Assistant (or should this be in the Elasticsearch Reference?)

Elasticsearch Reference:

- [x] Update upgrade steps in https://www.elastic.co/guide/en/elasticsearch/reference/7.0/setup-upgrade.html. See https://github.com/elastic/elasticsearch/pull/39944
- [x] Update machine learning considerations. See https://github.com/elastic/elasticsearch/pull/40006
- [x] Deprecate migration APIs. See https://github.com/elastic/elasticsearch/pull/40075

Kibana Guide:

- [x] Verify upgrade steps in https://www.elastic.co/guide/en/kibana/7.0/upgrade.html). See https://github.com/elastic/kibana/issues/33543 and https://github.com/elastic/kibana/pull/34121
- [x] Verify Upgrade Assistant screenshots and details (https://www.elastic.co/guide/en/kibana/6.7/upgrade-assistant.html)

Logstash Reference:

- [x] Verify upgrade steps in https://www.elastic.co/guide/en/logstash/7.0/upgrading-logstash.html See https://github.com/elastic/logstash/pull/10621
- [x] Update Breaking Changes docs https://www.elastic.co/guide/en/logstash/7.0/breaking-changes.html See https://github.com/elastic/logstash/pull/10632

Beats Platform Reference:

- [x] Verify upgrade steps in https://www.elastic.co/guide/en/beats/libbeat/7.0/upgrading.html See https://github.com/elastic/beats/pull/11378
- [x] Add release highlights: https://github.com/elastic/beats/pull/11621

Cloud:

- [x] Author ESS- and ECE- specific upgrade docs. See https://github.com/elastic/cloud/issues/27673

APM?

- [x] 7.0 Breaking changes, Release highlights, Migration assistant, ECS changes https://github.com/elastic/apm-server/pull/1940