Split $wgCentralAuthAutoMigrate into two settings
One for merging unattached accounts to an existing global one,
which was the original usage, and create
$wgCentralAuthAutoMigrateNonGlobalAccounts for globalizing unattached
The latter currently causes OOMs (T78727) in certain conditions on
the Wikimedia cluster, but it would still be a good idea to keep
the former enabled.