adjustments for php8, fixes #4111

lib/activities/Stream.php

lib/classes/Config.class.php

lib/classes/ForumHelpers.php

lib/classes/I18NString.php

lib/classes/JsonApi/Middlewares/StudipMockNavigation.php

lib/classes/MultiDimArrayObject.class.php

lib/classes/Request.class.php

lib/classes/SemClass.class.php

lib/classes/SemType.class.php

lib/classes/SessionDecoder.class.php

lib/classes/StudipArrayObject.class.php

lib/classes/StudipCachedArray.php

lib/classes/StudipPDO.class.php

lib/classes/StudipPDOStatement.php

lib/classes/UserDataAdapter.php

lib/classes/restapi/Response.php

lib/classes/sidebar/AttributesArrayAccessTrait.php

lib/filesystem/StandardFile.php

lib/filesystem/UnknownFileType.php

lib/models/DataField.class.php