* 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
Lee Hinman
Jim Ferenczi
Jim Ferenczi