MONGOID-5839 Fix eager-loading from STI subclasses (backport to 8.0) #3894

Job	Run time
ruby-3.1 driver-current mongodb-5.0 server	11m 34s
ruby-3.1 driver-current mongodb-5.0 replica_set	12m 53s
ruby-3.0 driver-current mongodb-5.0 server	10m 52s
ruby-3.0 driver-current mongodb-5.0 replica_set	13m 26s
jruby-9.3 driver-current mongodb-5.0 replica_set	23m 10s
ruby-2.7 driver-current mongodb-4.4 server	10m 38s
ruby-2.7 driver-current mongodb-4.4 replica_set	12m 40s
ruby-2.6 driver-current mongodb-4.0 server	1s
ruby-2.6 driver-current mongodb-4.0 replica_set	1s
ruby-2.6 driver-current mongodb-3.6 server	1s
ruby-2.6 driver-current mongodb-3.6 replica_set	1s
ruby-3.1 driver-master mongodb-5.0 replica_set	12m 54s
ruby-3.0 driver-stable mongodb-5.0 replica_set	12m 31s
ruby-2.6 driver-oldstable mongodb-4.0 replica_set	1s
ruby-2.6 driver-min mongodb-4.0 replica_set	1s
ruby-2.6 driver-min mongodb-3.6 server	1s
ruby-3.1 driver-current mongodb-5.0 server	11m 10s
ruby-3.1 driver-current mongodb-5.0 server	10m 57s
ruby-3.0 driver-current mongodb-5.0 server	11m 5s
ruby-3.0 driver-current mongodb-5.0 server	11m 17s
ruby-3.0 driver-current mongodb-5.0 server	10m 47s
ruby-2.7 driver-current mongodb-4.0 server	1s
ruby-2.6 driver-current mongodb-4.4 server	10m 47s
ruby-2.6 driver-current mongodb-4.2 server	1s
ruby-2.6 driver-current mongodb-4.2 server	1s
ruby-2.7 driver-current mongodb-5.0 server	11m 14s
jruby-9.3 driver-current mongodb-5.0 server	18m 6s
	3h 36m 11s

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