Breaking: #73445 - Remove flashmessage_compatibility.js from core 

See forge#73445

Description 

The flashmessage_compatibility.js has been removed from the core.

Impact 

Extensions which make use of TYPO3.Flashmessage JavaScript function will not work anymore.

Migration 

Use top.TYPO3.Notification.* to create flash messages within JavaScript for the TYPO3 backend.