6 files declare their use of MigrationConfigurationTrait in Drupal 9

MigrateUpgradeFormBase.php in core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeFormBase.php

MigrateUpgradeFormStepsTest.php in core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeFormStepsTest.php

MigrateUpgradeTestBase.php in core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php

migrate_drupal.module in core/modules/migrate_drupal/migrate_drupal.module

Provides migration from other Drupal sites.

StateFileExists.php in core/modules/migrate_drupal/tests/src/Kernel/StateFileExists.php

ValidateMigrationStateTestTrait.php in core/modules/migrate_drupal/tests/src/Traits/ValidateMigrationStateTestTrait.php