Breaking: #60559 - T3skin Backend Login Template File Moved

See Issue #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.