Update to Symfony 6 and PHP 8

Added

• added support for PHP 8 and 8.1
• added support for Symfony 5.4 and 6
• added use of attributes

Changed

• ControllerFilterEvent to ControllerEvent

Removed

• removed support for PHP 7 and lower
• removed support for Symfony 3 and 4
• removed annotations

v2.1.1: Merge pull request #20 from chadha/RadioVoterFixPsalm

RadioVoter fix psalm

v2.1.0: Merge pull request #19 from chadha/DEV-RatioVoterWithNullSession

make argument session optional, with on-invalid="null"