Breaking: #98158 - Update to Symfony 6

See forge#98158

Description

TYPO3 Core now ships with Symfony 6.1. Previously TYPO3 v11 used Symfony Components in version 5.4.

Impact

Some PHP code now might need to consider other types, especially regarding PHP classes which might be extended or used directly, where types for arguments are used.

One example is that all CLI Commands (used in custom extensions) now need to define an "int" as return type of the execute() method, otherwise the CLI command will not be executed anymore.

Affected installations

TYPO3 Installations with extensions making heavy use of Symfony components directly.

Migration

Functionality such as the CLI Commands can already put in place for TYPO3 versions prior to TYPO3 v12. It is recommended to use tools such as Rector to detect possible problems when having extensions interacting with Symfony Components directly.