That looks like a case of: "Shadow Terminator Artifacts"
It happens when ray-tracing over low poly geometry and the sharp edges of the geometry cause ridges in the shading. Blender has some settings to fix it.
In the Object Properties -> Shading settings try setting the Geometry and Shading offsets to 0.1 or higher. (Or just the shading if its before Blender 3.0)
You can also further reduce it by using Subdivision modifiers on the mesh.