GitLab 11.5 adds Elasticsearch 6, removes ES 5.5 support

GitLab 11.5 introduces support for Elasticsearch 6.x and removes support for Elasticsearch 5.5 and earlier. This upgrade is necessary due to breaking changes in Elasticsearch 6, specifically the removal of mapping types and the introduction of the 'join' datatype for parent-child relationships. The post details the technical implications of these changes on GitLab's indexing strategy, including the implementation of a custom 'type' field and the use of the 'join' datatype with routing by parent ID. Elasticsearch 5.6 is supported as a transitional version due to its early support for the 'join' datatype. This upgrade requires a reindex of Elasticsearch data.