mine was working perfectly and later I don’t know here another PHP came from (probably from setting up other project) so now i had 2 PHP and PHP7.4 version installed on my ubuntu. as @Gotzon comments I added php7.4 in my crontab.
*/5 * * * * /usr/bin/php7.4 -f /var/www/html/nextcloud/cron.php --define apc.enable_cli=1
Voillaa!!! It worked.