[32814] introduced a routine to split shared terms, which was run during the
regular WP database upgrade. This turned out to be problematic because plugins
are not loaded during the db upgrade (due to `WP_INSTALLING`), with the result
that plugins were not able to hook into the 'split_shared_term' action during
the bulk split. We work around this limitation by moving the term splitting
routine to a separate process, triggered by a wp-cron hook.
Props boonebgorges, Chouby, peterwilsoncc, pento, dd32.
Fixes#30261.
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