Make check_required_principal() case-insensitive
    
    service-del deletes services by DN and LDAP DNs are compared
    case-insensitive. Make check_required_principal() compare the
    service name case insensitive.
    
    Fixes: https://pagure.io/freeipa/issue/8308
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>