update to 2.7.0
    add dependency on doctrine/event-manager
    add dependency on doctrine/persistence
    raise dependency on doctrine/annotations 1.8
    raise dependency on doctrine/cache 1.9.1
    raise dependency on doctrine/collections 1.5
    raise dependency on doctrine/common 2.11
    raise dependency on doctrine/dbal 2.9.3
    raise dependency on doctrine/instantiator 1.3
    allow Symfony 5
    symfony/yaml is optional