I have a high-powered copter and want to limit the flight speed to a certain value, let’s say e.g. 3 m/s. On my search of finding a solution I came across the ANGLE_MAX parameter which sets an upper limit for the lean angle in any direction and flight mode. (Please correct me if I am wrong at that point.)
Now, my question is what is the relationship between the lean angle and the flight speed?

I would say there is no fixed relation between lean angle and speed.
The lean angle creates a horizontal force/thrust component from the vertical/lift thrust. This horizontal force accelerates the copter until the air resistance and the horizontal force are equal. So you would have to know the drag coefficient of your copter to know which lean angle/horizontal force results in which maximum velocity.
This is all regarding airspeed, not groundspeed. If you limit your copter to 3m/s airspeed and you have 3m/s or more headwind, the copter will go nowhere.

Thanks, @TunaLobster and @count74!
My background is not so much in fluid dynamics but the drag coefficient is a good hint.

If I take the definition from Wikipedia, it seems that it’s possible to estimate this for a given copter as the area A can be roughly assumed as being the spread of the arms including the rotors. Rho would be the density of air. But what about u and the drag force?