Grader report generate an exception with PHP 8.0

Hello,

The grader report generate an exception with PHP 8.0.

The PHP error :

[04-Aug-2022 12:48:12 Europe/Paris] Default exception handler: Exception - Unsupported operand types: string * int

Debug: 
Error code: generalexceptionmessage

• line 492 of /grade/report/grader/lib.php: TypeError thrown
• line 158 of /grade/report/grader/index.php: call to grade_report_grader->load_users()

PHP 8.0 seems to not appreciate the operation string*int.

Proposed (tested and working) solution :

diff grade/report/grader/lib.php.old  grade/report/grader/lib.php.new
2015c2015
<         return $this->get_pref('studentsperpage');
—
>         return intval($this->get_pref('studentsperpage'));

Thanks a lot for all your work on Moodle !

Baptiste