6 files declare their use of ConnectionNotDefinedException in Drupal 9

DbCommandBaseTest.php in core/modules/system/tests/src/Kernel/Scripts/DbCommandBaseTest.php

Contains \Drupal\Tests\system\Kernel\Scripts\DbCommandBaseTest.

InstallCommand.php in core/lib/Drupal/Core/Command/InstallCommand.php

ServerCommand.php in core/lib/Drupal/Core/Command/ServerCommand.php

SqlBase.php in core/modules/migrate/src/Plugin/migrate/source/SqlBase.php

Tasks.php in core/lib/Drupal/Core/Database/Driver/mysql/Install/Tasks.php

TestDatabase.php in core/lib/Drupal/Core/Test/TestDatabase.php