Update CHANGELOG-EE.md for 12.2.3-ee

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CHANGELOG-EE.md

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+## 12.2.3
+
+### Security (2 changes)
+
+- Limit number of jobs in running pipelines for the past hour on per plan basis. !1182
+- Filter out old system notes for epics in notes api endpoint response.
+
+
 ## 12.2.2
 ### Security (2 changes)