System.Text.Json instances of polymorphic types emit type discriminator when serialized as `object` types

[eiriktsarpalis]

Creating an issue to track

https://github.com/dotnet/runtime/blob/main/src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/PolymorphicTests.CustomTypeHierarchies.cs#L937-L943

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Issue Details

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Creating an issue to track

https://github.com/dotnet/runtime/blob/main/src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/PolymorphicTests.CustomTypeHierarchies.cs#L937-L943

Author:	eiriktsarpalis
Assignees:	-
Labels:	area-System.Text.Json
Milestone:	-