Bump max_total_records per Spanner's 80k mutation limit

[data-sync-user]

We currently have a traditionally small max_total_records client limit (compared to the older mysql/postgres implementations) due to Spanner’s original 20k mutation per commit limit. Spanner has improved this limit over time, now supporting 80k mutations.

We should bump the max_total_records accordingly so clients can potentially commit larger, fewer batches. This should be roughly be 4x the current limit of 1664 – the mutation math documentation should also be recalculated/updated.

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