Heh, yeah... C++ is much more common on the engine development end than on the game development end, but you can kind of use whatever you want with Godot through GDExtension. This should set you in the right direction:
C++ is orders of magnitude more complex than C; you should be able to get the fundamentals quite easily. C# isn't particularly a good choice for a game engine: the main benefit is JIT & runtime flexibility; modding C# games is a very pleasant experience, but the performance can hurt sometimes.
If you go here and sort by release date https://en.wikipedia.org/wiki/List_of_game_engines , you'll find that a lot of modern engines use C++, including some of the most popular like Godot and Unreal. C is still often used for some low level & platform specific stuff, but there's a TON of C++ in the games industry
Comments
https://github.com/godotengine/godot-cpp
But, in any case, switching to C# once you've learned C++ is easy. The skills in programming are super transferable.
Would kinda be like using a hydraulic press to hammer in a nail 😅
I'm hope I'm wrong about that