Issue #3399645 by znerol, smustgrave: Use structured DSN instead of URI in system.mail mailer_dsn

core/lib/Drupal/Core/Mail/Plugin/Mail/SymfonyMailer.php

core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php

...ui/tests/src/Functional/MigrateUpgradeExecuteTestBase.php

core/modules/system/config/install/system.mail.yml

core/modules/system/config/schema/system.schema.yml

core/modules/system/migrations/d7_system_mail.yml

core/modules/system/system.post_update.php

...tests/src/Functional/Update/MailDsnSettingsUpdateTest.php

.../src/Kernel/Migrate/d7/MigrateSystemConfigurationTest.php

...ts/Drupal/FunctionalTests/Installer/InstallerTestBase.php

core/tests/Drupal/KernelTests/KernelTestBase.php

core/tests/Drupal/Tests/Core/Mail/MailManagerTest.php

.../tests/Drupal/Tests/Core/Mail/Plugin/Mail/PhpMailTest.php