refs https://github.com/TryGhost/Ghost/pull/14197
- Moved from updating the last_seen_at value "at most every 24h" to "at most every UTC day".
- It will simplify explaining the following behavior: a publication is set in UTC-10, a user visits at 2pm on Monday and at 1pm on Tuesday, the last_seen_at value is still Monday.
- There is no way to go around the above issue due to the technical constraint of updating the `last_seen_at` value at most once a day.
- This might create database write spikes at midnight UTC
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