Feature: #92616 - Bootstrap v5

See Issue #92616

Description

TYPO3s Backend is powered with Bootstrap v5 (https://getbootstrap.com).

Previous versions ran with Bootstrap v3. A lots of changes and improvements are included, especially regarding simplification of building Layouts and Containers.

Backend modules which relied on Twitter Bootstrap v3 markup, or functionality in JavaScript, will have a slightly different and improved look & feel.

Impact

It is important during updates of custom backend modules to study the Bootstrap Migration guidelines and adapt accordingly:

TYPO3 Core still includes some legacy styling and functionality to provide compatibility, but backend markup and code will be adapted during further TYPO3 v11 development to simplify the HTML and CSS code shipped with TYPO3 Core.