[2.0.x] $request->get may no longer return array as of Symfony 6.0 (BadRequestException)

Problem/Motivation

I encounter the following error in the latest dev branch:

The website encountered an unexpected error. Please try again later.InvalidArgumentException: Expected a scalar value as a 2nd argument to "Symfony\Component\HttpFoundation\InputBag::get()", "array" given. in Symfony\Component\HttpFoundation\InputBag->get() (line 31 of /var/www/vendor/symfony/http-foundation/InputBag.php). Drupal\facets\Plugin\facets\url_processor\QueryString->initializeActiveFilters() (Line: 59)

Steps to reproduce

1. Run AggregatedFieldTest test

Proposed resolution

In symfony 6, the InputBag::get(), no longer accepts arrays as default values, so we need to change it to string.

Remaining tasks

User interface changes

API changes

Data model changes