Check whether `Kernel::System::CustomerUserCustomerSearch()` should support the attribute Valid

          When looking at the code I found that `Kernel::System::CustomerUser::LDAP::CustomerSearch()` looks funny. The documentation of  `Kernel::System::CustomerUserCustomerSearch()`says:

    # username search
    my %List = $CustomerUserObject->CustomerSearch(
        UserLogin => '*some*',
        Valid     => 1,         # (optional) default 1
    );

The setting of Valid is checked in Kernel::System::CustomerUser::DB, but I didn't find such a check in Kernel::System::CustomerUser::LDAP. It is not obvious whether this is a glitch in the implementation or whether the documentation should be enhanced to cover the special case.

Originally posted by @bschmalhofer in https://github.com/RotherOSS/otobo/issues/2583#issuecomment-1786816103