Register event listener to remove migration table from comparison schema

[wmouwen]

The PR doctrine/orm#11374 is created to add a new event to doctrine/orm: ToolEvents::postGenerateComparisonSchema. This event enables doctrine/migrations#1418 where the migration version table can be removed from the generated comparison schema.

This PR registers the new event listener with the Symfony container, fixing #1406.

> bin/console doctrine:schema:validate

Mapping
-------
                                                                                                             
 [OK] The mapping files are correct.                                                                         
                                                                                                             

Database
--------
                                                                                                             
 [OK] The database schema is in sync with the mapping files.                                                 
                                                                                                             

PR relates to an attempt to fix doctrine/migrations#1406. It requires doctrine/orm#11374 and doctrine/migrations#1418.