[TASK] Update UPGRADE.md

UPGRADE.md

@@ -2,6 +2,7 @@
 - `youtubeLightbox.js` (deprecated since 4.4.0) using magnific-popup and jQuery removed in favor of `sgYoutubeLightbox.js` (vanilla JS).
 - Extension `project_theme_lightbox` required starting with version 5.0.0.
 - Dropped TYPO3 9 Support
+- Dropped php 7.3 Support
 
 ## Version 4.4  ```project_theme_lightbox``` integration
 - The magnific popup integration is deprecated and will be removed in later versions.