[!!!][FEATURE] Set Symfony 6.1 as minimum requirement

This change raises the used Symfony Components
to at least version Symfony 6.1.

The requirement for "typo3/symfony-psr-event-dispatcher-adapter"
is removed, as this was for support for allowing
symfony4 AND symfony5 with the same TYPO3 major version.

A few important highlights:
* Symfony DI now only accepts objects or null to be
 returned by $container->get('myService')
* Lots of typehints were added, this is mainly an issue
  if extensions extend Symfony classes
* PHP 8.1+ requirements (which we already have in place for TYPO3 v12)

See all features within Symfony 6.1:
* https://symfony.com/blog/category/living-on-the-edge/6.1

Symfony 6 is supported until November 2027 (once we
upgrade to Symfony 6 LTS, which will happen in November 2023)
unlike Symfony 5 LTS which has an EOL on November 2025.

Used commands:

> composer req composer/composer:^2.3.0 -W --dev
> composer remove typo3/symfony-psr-event-dispatcher-adapter
> composer remove --no-update ...