Using soft reserved words (numeric, enum) could break in the future

This was detected as part of MDL-73587 (PHPCompatibility complete run against codebase).

• 'numeric' is a soft reserved keyword as of PHP version 7.0 and should not be used to name a class, interface or trait or as part of a namespace (namespace declaration)
• 'enum' is a soft reserved keyword as of PHP version 8.1 and should not be used to name a class, interface or trait or as part of a namespace (namespace declaration)

Note this is not critical and doesn't lead to any problem or PHP notice/warning/deprecation... just they are reserved and we should try to stop using them.

Cases:

• lib/php-enum/src/Enum.php : 7 & 22
• mod/lesson/classes/local/numeric/helper.php : 26

Ciao