Update the license headers for Nextcloud 19

AUTHORS

apps/accessibility/lib/AccessibilityProvider.php

.../accessibility/lib/Controller/AccessibilityController.php

apps/accessibility/lib/Controller/ConfigController.php

apps/admin_audit/lib/AppInfo/Application.php

...ederation_api/lib/Controller/RequestHandlerController.php

apps/comments/lib/AppInfo/Application.php

apps/comments/tests/Unit/Notification/ListenerTest.php

apps/comments/tests/Unit/Notification/NotifierTest.php

apps/dav/appinfo/app.php

apps/dav/appinfo/v1/publicwebdav.php

apps/dav/lib/AppInfo/Application.php

apps/dav/lib/AppInfo/PluginManager.php

apps/dav/lib/BackgroundJob/EventReminderJob.php

apps/dav/lib/BackgroundJob/UploadCleanup.php

apps/dav/lib/CalDAV/Activity/Backend.php

apps/dav/lib/CalDAV/Activity/Provider/Calendar.php

apps/dav/lib/CalDAV/CalDavBackend.php

apps/dav/lib/CalDAV/Calendar.php

apps/dav/lib/CalDAV/CalendarObject.php