Index lifecycle management [Phase 1]: Adapt gitlab-elasticsearch-indexer for ILM

In this issue we should recreate the logic from #373986 (closed) in gitlab-elasticsearch-indexer

On writes, the gitlab-elasticsearch-indexer should send a _bulk request to delete document from all read indices.

Add a flag (maybe ilm_mode) for supporting > 1 index. Indexer asks Elasticsearch for the list of aliases to have the list of read-only and the write index.