elk: automatically limit log retention

The index retention should be automatically managed to limit the number of opened indexes.

There are 2 types of indexes on the cluster:

• functional logs indexes (task-.* swh_workers-.*
• system log indexes (apache_logs-.* systemlogs-.*)

The rules can be :

• both types of indexes: close indexes older than 6 months
• system logs: delete indexes older than 12 months

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Migrated from T3221 (view on Phabricator)