Fixed #28543 -- Prevented ManyToManyField.value_from_object() from being lazy.

Previously, it was a QuerySet which could reevaluate to a new value if the
model's data changes. This is inconsistent with other Field.value_from_object()
methods.

This allows reverting the fix in the admin for refs #27998.

tests/model_fields/models.py

@@ -360,6 +360,10 @@ class AllFieldsModel(models.Model):
     gr = GenericRelation(DataModel)
 
 
+class ManyToMany(models.Model):
+    m2m = models.ManyToManyField('self')
+
+
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