Use flattened field type instead of object for neos_fulltext_parts

[daniellienert]

The field neos_fulltext_parts stores the fulltext path for every aggregateRoot document with the childnode identifiers as keys. This easily can lead to mapping explosion. Starting with Elasticsearch > 7.3, you can configure:

  properties:
    'neos_fulltext_parts':
      search:
        elasticSearchMapping:
          type: flattened
        indexing: ''

to avoid this.

This should be the default when 7.x is the minimal supported version.

Note: as per https://www.elastic.co/guide/en/elasticsearch/reference/7.5/release-highlights-7.3.0.html#_new_flattened_field_type this type is only available "with the default distribution of Elasticsearch." – whatever that means.