Update prepare_async_index migration helper to avoid subtransaction

What does this MR do?

safe_find_or_create_by! creates a savepoint if already in transaction, which is usually the case for migrations. This updates the helper to use upsert instead in order to avoid subtransactions.

safe_find_or_create_by! creates a savepoint if already in transaction, which is usually the case for migrations. This updates the helper to use 'find_or_create_by!` instead in order to avoid subtransactions.

find_or_create_by! is not atomic, but given this helper is used only on migrations and they are never executed in parallel, this is fine.

Also changes the helper to update index definition if the record already exists and there is new definition, something we state in the method comment but were not actually doing.

Related to #339667 (closed).

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