Update CHANGELOG-EE.md for 12.9.3-ee

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CHANGELOG-EE.md

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+## 12.9.3 (2020-04-14)
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+### Security (1 change)
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+- Fix filename bypass when uploading NuGet packages.
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+
 ## 12.9.2 (2020-03-31)
 
 ### Fixed (4 changes)