Since dates are stored as plain text, you won't be able to compare them to the current date. In this case, the >= operator in your LDAP filter can only compare strings instead of comparing the actual dates.
To do what you need, you'll need to get the value of the terminationDate attribute of each user, convert it to a DateTime structure, and then compare it to the target date.
Can you provide the format in which dates are stored in the terminationDate attribute so we could provide you a script? Just a couple of examples of the dates stored in the attribute will be enough.