* Remove the _all metadata field
This change removes the `_all` metadata field. This field is deprecated in 6
and cannot be activated for indices created in 6 so it can be safely removed in
the next major version (e.g. 7).
We use `:` for cross-cluster search (eg `cluster:index`), therefore, we should
not allow the ambiguity when allowing cluster or index names.
Relates to #23892
* 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
