Breaking: #60559 - T3skin Backend Login Template File Moved 

See forge#60559

Description 

The HTML template file for the backend login screen was moved from EXT:t3skin to EXT:backend.

Impact 

Broken login screen.

Affected installations 

A TYPO3 instance is affected if a 3rd party extension uses file EXT:t3skin/Resources/Private/Templates/login.html

Migration 

Use file EXT:backend/Resources/Private/Templates/login.html instead or refactor the affected extension to free it from the dependency to this core internal file.