PHP Deprecation Notice: Use of “self” in callables is deprecated

The function vxcf_form::clean() is using the string self as a callables parameter which will be removed in a future PHP version, see https://php.watch/versions/8.2/partially-supported-callable-deprecation.

Possible patch:

- return array_map( array('self','clean'), $var );
+ return array_map( array(vxcf_form::class,'clean'), $var );

Would be great to get this fixed to avoid such PHP notices on current PHP versions. Thanks!