This commit restores a noop version of the AllFieldMapper that is instanciated only
for indices created in 6x. We need this metadata field mapper to be present in this version
in order to allow the upgrade of indices that explicitly disable _all (enabled: false).
The mapping of these indices contains a reference to the _all field that we cannot remove
in 7 so we'll need to keep this metadata mapper in 7x. Since indices created in 6x will not
be compatible with 8, we'll remove this noop mapper in the next major version.
Closes#37429
5dcc805dc9