Update CHANGELOG-EE.md for 13.1.3-ee

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CHANGELOG-EE.md

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+## 13.1.3 (2020-07-06)
+
+### Security (1 change)
+
+- Maven packages upload endpoint is now properly using the uploaded file set by middleware.
+
+
 ## 13.1.2 (2020-07-01)
 ### Security (2 changes)

ee/changelogs/unreleased/security-use-multipart-uploaded-file-in-maven-packages.yml

----
-title: Maven packages upload endpoint is now properly using the uploaded file set
-  by middleware
-merge_request:
-author:
-type: security