Clearing the cache indices can be done via GET and POST. As GET should
only support read only operations, this removes the support for using
GET for clearing the indices caches.
* Migrate migration docs from 6.0 to 7.0
Since we only keep one version of migration docs and master is now on 7.0, we
should migrate these so breaking changes can be added in the right place.
* Remove release notes as well
They link to the migration guides, so they have to go.
* Add placeholder notes for 7.0 so doc build is happy
Nik Everett
Alexander Reelsen
Simon Willnauer
Lee Hinman