Reindex Elasticsearch merge request to update integer with long type

Arturo Herrerorequested to merge
502817-reindex-elasticsearch-merge-requests-to-update-integer-to-long-type into master

Description

This Elasticsearch migration updates the merge request mapping to match the PostgreSQL database types with the Elasticsearch documents indexed from integer to long.

We cannot use MigrationUpdateMappingsHelper because Elasticsearch doesn't update the mappings with existing data (demo !182255 (comment 2365206340)). So, we need to use a zero-downtime reindex migration using ReindexingTask.

Reference: https://docs.gitlab.com/development/search/advanced_search_migration_styleguide/#zero-downtime-reindex-migration

Related to #502817 (closed).

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