Ignore unknown sequences in sequence fix migration

What does this MR do and why?

This backports !160609 (merged) to 17-2-stable-ee to fix an upgrade problem described in #474293 (closed).

GitLab 17.2.1 included a migration to fix the wrong owners of sequences (!160139 (merged)). Unfortunately due to a bug in the sequences_owned_by query described in #468541 (comment 2013158432), some sequences may have been assigned the wrong owner and not automatically dropped when its associated table was dropped.

For example, for one customer geo_reset_checksum_events_id_seq was assigned to the wrong owner (p_ci_builds.partition_id), so when geo_reset_checksum_events was dropped in GitLab 17.0 via !151938 (merged), the sequence remained in the database. We could remove these unused sequences, but that should probably be done by a different migration (UPDATE: done in !160624 (merged)). For now, ignoring these sequences is the safest way to handle this.

Relates to #474293 (closed)

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