Mappings: Allow _version to use `disk` as a doc values format.

VersionFieldMapper.defaultDocValuesFormat claims that the default is `disk`.
This is not used to choose the DV format in the index but for mappings
serialization in order to know when the _version doc values format is
different from the default format. This made it impossible to use the `disk`
doc values format since mappings would never retain that information at
serialization time.

Close #6523