Have you read the official Godot License’s description? It can be found here: https://godotengine.org/license

That's a good point, I'd assumed since they're using the MIT License that you only have to reference the license with the source code, but I see they state otherwise in the link provided by Dark Dimension. I'm going to go with the following recommendation there and place a link in all my itch.io game descriptions that use Godot (and also in credits if the game has in-game credits).

 "The Godot Engine developers consider that a link to this page (godotengine.org/license) in your game documentation or credits would be an acceptable way to satisfy the license terms."

For what it's worth, while the MIT license says you always have to include it if you distribute (part of) a software project that uses it, many developers don't do that in practice. Think of all those websites that use jQuery and never mention it once. If in doubt, provide credit as requested by the creators, like technicat says.