Advanced Search unsupported migration should be a preflight check

What does this MR do and why?

This change adds a new preflight check into the Elastic::MigrationWorker. If it detects that the customer is running an unsupported Elasticsearch version (6.x in this case), it will pause indexing to prevent data loss. This essentially replicates the one-time migration we've had, but uses the new unsupported_version? method.

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Related to #363610 (closed)