Upgrading a project with Universal RP throws deserialization errors.

To reproduce:

1. Create a project with Universal RP 6.9 in 2019.2 or older version
2. Open the project in Unity 2019.3 or a newer version

Expected result: Unity opens with no errors
Actual result:
ArgumentException: Can not deserialize (UnityEditor.Experimental.Rendering.LightweightPipeline.LightWeightPBRSubShader), type is invalid
UnityEditor.Graphing.SerializationHelper.Deserialize[T] (UnityEditor.Graphing.SerializationHelper+JSONSerializedElement item, System.Collections.Generic.Dictionary`2[TKey,TValue] remapper, System.Object[] constructorArgs) (at Library/PackageCache/com.unity.shadergraph@7.0.1/Editor/Data/Util/SerializationHelper.cs:103)
UnityEditor.Graphing.SerializationHelper.Deserialize[T] (System.Collections.Generic.IEnumerable`1[T] list, System.Collections.Generic.Dictionary`2[TKey,TValue] remapper, System.Object[] constructorArgs) (at Library/PackageCache/com.unity.shadergraph@7.0.1/Editor/Data/Util/SerializationHelper.cs:153)
UnityEditor.Experimental.AssetImporters.ScriptedImporter:GenerateAssetData(AssetImportContext)

{}
UnityEditor.Experimental.AssetImporters.ScriptedImporter:GenerateAssetData(AssetImportContext)

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