If you have the cable for the old power adapter you can use that with the new phone.
If you need to or want to buy a new adapter to go with the new cable, see: Buy 20W USB-C Power Adapter - Apple
Also see the following from Buy iPhone 11 - Apple
As part of our efforts to reach our environmental goals, iPhone 11 does not include a power adapter or EarPods. Included in the box is a USB‑C to Lightning cable that supports fast charging and is compatible with USB‑C power adapters and computer ports.
We encourage you to re‑use your current USB‑A to Lightning cables, power adapters, and headphones which are compatible with this iPhone. But if you need any new Apple power adapters or headphones, they are available for purchase.