Your site has been upgraded to PHP 7 at least a year ago already.
We ran a patched version of PHP 7 that allowed people to use mysql_ style functions on PHP 7, despite these functions not actually existing in the official PHP 7 code. It appears that this patch has not been carried over to PHP 8.
So I think you now urgently need to do what you should have done 8 years ago already: rewrite your code to use mysqli_ functions instead of mysql_.