This change adjusts the install & upgrade routines so all of ID-based database columns in the multisite database tables are `unsigned`, bringing them up-to-speed with ID-based columns in single-site tables.
Additionally, the `$wp_db_version` number is bumped, and the `pre_schema_upgrade()` upgrade function is modified to accommodate & use that new version.
Follow-up to [10852].
Props spacedmonkey, johnjamesjacoby.
Fixes#40418.
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