Change default behavior of container_base_class to use PhpContainer

Problem/Motivation

In #3583505: Use Symfony PhpDumper instead of a serialized array container structure we introduced the ability to dump the container to disk. All new sites are using this but existing sites will use the array based container unless they've updated their settings.php. In Drupal 12 will will change this so that existing sites will use the PHP dumped container and have to set the container_base_class in order to use the array container.

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet