Advanced Search: Index label_ids for merge requests

Arturo Herrerorequested to merge
label-facet-merge-request into master

Description

This adds a new Advanced search migration to add label_ids to the merge requests.

The new label_ids field is a keyword, intended to store an array of label ids. This is very similar to what we have done for issues !96153 (merged) without the backfilling that we will complete as part of #464302 (closed).

Related to #452399 (closed).

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