If they are carpenter ants, they may be eating their way through the wood, contributing to the damage caused by discharge from the former AC installed here. If they're "regular" ants, they are likely just finding their way through holes that have been rotted out by the regular drip of moisture from the AC unit. The ants may also be coming from neighbors apartments in addition to coming in through this opening. You can put down traps or chemicals to kill the ants, but you may be facing an ongoing battle, as new ants will come to replace the now dead ones.
You're right that there's not much you can do about the structural issue in the window frame - you have to get building maintenance to do that work (whether they do it or contract it out is up to them). Building maintenance should be interested in protecting the building, however, as you've noted, it's rent-controlled, so the owners may not be making a lot of profit on it, so it may, unfortunately, be lower on their maintenance list.
If this continues without response from management and becomes unlivable for you, you may ultimately have to get the city health department and or building commission involved in forcing the owner's hand in making repairs. Note that this may have other consequences - they can't raise your rent, but they might not renew your lease and may become unresponsive to other maintenance requests.
A less "nuclear" option might be to simply apply some caulk to the inside to plug up the holes to keep ants and moisture out and keep air from flowing through - heat out in the summer, heat in in the winter. This isn't a "proper" fix and not one you should do on your own property, but it is a stop-gap measure that may be workable if the building owner just isn't responding.
