Update Symfony

[nfebe]

Update symfony and affected packages.

phpList/core currently depends on symfony 3.4.37 which is several versions behind the latest version and is not supported.

For this to be done, phpunit/dbunit first has to be removed as mentioned in #324 as it requires various packages that have been replaced by newer versions of symfony/symfony see;

  Problem 1
    - phpunit/dbunit[3.0.2, ..., 3.0.3] require symfony/yaml ^3.0 || ^4.0 -> satisfiable by symfony/yaml[v3.0.0, ..., v3.4.47, v4.0.0, ..., v4.4.20].
    - phpunit/dbunit[3.0.0, ..., 3.0.1] require symfony/yaml ^3.0 -> satisfiable by symfony/yaml[v3.0.0, ..., v3.4.47].
    - Only one of these can be installed: symfony/yaml[v3.0.0, ..., v3.4.47, v4.0.0, ..., v4.4.20], symfony/symfony[v5.2.5]. symfony/symfony replaces symfony/yaml and thus cannot coexist with it.
    - Root composer.json requires symfony/symfony ^5.2.5 -> satisfiable by symfony/symfony[v5.2.5].
    - Root composer.json requires phpunit/dbunit ^3.0.0 -> satisfiable by phpunit/dbunit[3.0.0, 3.0.1, 3.0.2, 3.0.3].

[TatevikGr]

Resolved: #331