ParameterHandler Versions Save

• Drop support for old PHP versions. The min version is now PGP 7.4.
• Drop support for old Symfony versions. The min version is now Symfony 5.4
• Mark symfony/yaml 7 as supported

Add support for Symfony 6

• Marked symfony/yaml 4 as supported to be compatible with Symfony 4

• Mark symfony/yaml 3 as supported to be compatible with Symfony 3
• Dropped support for symfony/yaml 2.2 and older (which are long unmaintained)
• Added testing on PHP 7

• Removed usage of a deprecated way to use the Yaml parser
• Added a more detailed exception message when the top-level key is missing

• Move most of the logic to a Processor class which does not depend on the composer event and package. Ref #30
• Add the support of existing empty file for Capifony compatibility
• Add the support of multiple managed files
• Preserve other top-level keys than the configured one in the file
• Add a rename map used to rename parameters when updating the parameters file
• Add the possibility to use another top-level key than parameters