* Adds a shared note that the migration APIs are mainly intended for internal use by Kibana's Upgrade Assistant.
* Updates the feature migration API docs to use updated "migration" terminology.
* Removes some references to major versions from the deprecation API docs.
Removes `testenv` annotations and related code. These annotations originally let you skip x-pack snippet tests in the docs. However, that's no longer possible.
Relates to #79309, #31619
* Add stubs for get API
* Add stub for post API
* Register new actions in ActionModule
* HLRC stubs
* Unit tests
* Add rest api spec and tests
* Add new action to non-operator actions list
The Migration Assistance API has been functionally replaced by the
Deprecation Info API, and the Migration Upgrade API is not used for the
transition from ES 6.x to 7.x, and does not need to be kept around to
repair indices that were not properly upgraded before upgrading the
cluster, as was the case in 6.
