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

[lcawl]

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):

• Update upgrade steps. See 7.0 upgrade #242
• Verify list of "notable" breaking changes.
• Address feedback in 7.0 stack upgrade docs feedback #226
• Add a topic that discusses the benefits of upgrading. See [DOCS] Added upgrade intro #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:

• Update upgrade steps in https://www.elastic.co/guide/en/elasticsearch/reference/7.0/setup-upgrade.html. See [DOCS] First pass at upgrade updates for 7.0. elasticsearch#39944
• Update machine learning considerations. See [DOCS] Simplify ML upgrade step elasticsearch#40006
• Deprecate migration APIs. See Remove Migration Upgrade and Assistance APIs elasticsearch#40075

Kibana Guide:

• Verify upgrade steps in https://www.elastic.co/guide/en/kibana/7.0/upgrade.html). See 7.0 upgrade docs need to be updated kibana#33543 and [DOCS] Updates 7.0 upgrade guide kibana#34121
• Verify Upgrade Assistant screenshots and details (https://www.elastic.co/guide/en/kibana/6.7/upgrade-assistant.html)

Logstash Reference:

• Verify upgrade steps in https://www.elastic.co/guide/en/logstash/7.0/upgrading-logstash.html See [Doc] Update instructions for 7.0 upgrade logstash#10621
• Update Breaking Changes docs https://www.elastic.co/guide/en/logstash/7.0/breaking-changes.html See [WIP] Update breaking changes doc for 7.0 logstash#10632

Beats Platform Reference:

• Verify upgrade steps in https://www.elastic.co/guide/en/beats/libbeat/7.0/upgrading.html See Add upgrade docs beats#11378
• Add release highlights: Add release highlights beats#11621

Cloud:

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

APM?

• 7.0 Breaking changes, Release highlights, Migration assistant, ECS changes [Docs] 7.0 Breaking changes, Release highlights, Migration assistant, ECS changes apm-server#1940