6 files declare their use of Connection in Drupal 9

Connection.php in core/modules/system/tests/modules/database_statement_monitoring_test/src/mysql/Connection.php

Connection.php in core/modules/system/tests/modules/driver_test/src/Driver/Database/DrivertestMysql/Connection.php

Connection.php in core/modules/system/tests/modules/driver_test/src/Driver/Database/DrivertestMysqlDeprecatedVersion/Connection.php

ConnectionTest.php in core/tests/Drupal/Tests/Core/Database/Driver/mysql/ConnectionTest.php

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

TasksTest.php in core/tests/Drupal/Tests/Core/Database/Driver/mysql/install/TasksTest.php