[TASK] Update compatibility and codestyle to TYPO3 9 LTS

.gitlab-ci.yml

Build/.php_cs.php

Build/phpstan.neon

Classes/Command/MigrateCalEntriesToCalendarCommand.php

Classes/Domain/Repository/CalRepository.php

Classes/Service/MigrateCalToCalendarFoundationService.php

...s/Traits/MigrateCalCalendarsToCalendarFoundationTrait.php

...ses/Traits/MigrateCalEntriesToCalendarFoundationTrait.php

Classes/Update/MigrateCalendarDatesUpdateWizard.php

.../Update/MigrateToCalendarFoundationLegacyUpdateWizard.php

Classes/Update/MigrateToCalendarFoundationUpdateWizard.php

Configuration/Commands.php

Configuration/TCA/Overrides/tx_calendar_calendar.php

Configuration/TCA/Overrides/tx_calendar_deviation.php

Configuration/TCA/Overrides/tx_calendar_entry.php

Configuration/TCA/Overrides/tx_calendar_exception.php

Configuration/TCA/Overrides/tx_calendar_exception_group.php

...s/Functional/Fixtures/post_migration_calendar_entries.csv

Tests/Functional/Service/MigrateCalEntriesServiceTest.php

...ice/MigrateCaCalendarsToCalendarFoundationServiceTest.php