Classification Changes 2.1 Change Option precedence within 'required' classes

Alter the option precedence of classes from require-client-classes as described https://gitlab.isc.org/isc-projects/kea/-/wikis/Designs//Potential-Classification-Changes#21-change-option-precedence-within-required-classes

Among classes added via require-client-classes, option precedence is shared-network over subnet over pool. This change would reverse the order to pool over subnet over shared-network making it match that of explicitly specified options.