[Postgress] Variable to config: search_active_modules

Problem/Motivation

This introduced PostgreSQL failure:
- https://www.drupal.org/pift-ci-job/1147872
- https://www.drupal.org/pift-ci-job/1147909

Drupal\Tests\search\Kernel\Migrate\d7\MigrateSearchPageTest::testSearchPage
Failed asserting that Array &0 (
    'rankings' => Array &1 (
        'promote' => 0
        'relevance' => 2
        'sticky' => 0
        'views' => 0
        'comments' => 4
    )
) is identical to Array &0 (
    'rankings' => Array &1 (
        'comments' => 4
        'promote' => 0
        'relevance' => 2
        'sticky' => 0
        'views' => 0
    )
).

/var/www/html/core/modules/search/tests/src/Kernel/Migrate/d7/MigrateSearchPageTest.php:43
/var/www/html/core/modules/search/tests/src/Kernel/Migrate/d7/MigrateSearchPageTest.php:79

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet