MONGOID-5839 Fix eager-loading from STI subclasses (backport to 8.1) #3893

Job	Run time
ruby-3.2 driver-current mongodb-6.0 replica_set	13m 4s
ruby-3.1 driver-current mongodb-6.0 replica_set	13m 29s
ruby-3.0 driver-current mongodb-6.0 replica_set	13m 33s
ruby-3.1 driver-master mongodb-6.0 replica_set	13m 7s
ruby-3.0 driver-stable mongodb-6.0 replica_set	12m 53s
ruby-3.3 driver-current mongodb-7.0 server	11m 18s
ruby-3.2 driver-current mongodb-7.0 server	11m 42s
ruby-3.2 driver-current mongodb-7.0 server	10m 59s
ruby-3.1 driver-current mongodb-7.0 server	11m 37s
ruby-3.1 driver-current mongodb-6.0 server	11m 27s
ruby-3.0 driver-current mongodb-6.0 server	11m 25s
ruby-3.0 driver-current mongodb-6.0 server	11m 48s
ruby-2.7 driver-current mongodb-6.0 server	11m 21s
jruby-9.3 driver-current mongodb-6.0 server	19m 12s
ruby-3.1 driver-current mongodb-5.0 replica_set	13m 42s
ruby-2.7 driver-current mongodb-4.4 replica_set	13m 1s
	3h 23m 38s

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