Assets from Text Core that were created while using the UI Toolkit package are not converted to be compatible with 2021.2 when using the conversion tool.

Steps to reproduce:
1. Create an empty project with Unity 2020.3
2. Add the UI Toolkit package (version preview.14) by typing "com.unity.ui" on the Package Manager (add package from git URL option).
3. Create a PanelSettings somewhere in the project. Notice there's now a folder "UI Toolkit/Resources" with subfolders containing different Text Core assets (FontAsset, SpriteAsset, TextColorGradient, TextSylesheet).
4. Close Unity.
5. Open either a recent Unity 2021.2 or trunk (2022.1) and open the project created in step 1.
6. Select Window > UI Toolkit > Package Asset Converter and click the "Convert" button.

Expected result: the Text Core assets inside the "UI Toolkit/Resources" subfolders are recognized as valid assets.
Actual result: the Text Core assets inside the "UI Toolkit/Resources" subfolders are shown as "missing (Mono Script)" and are therefore not valid.