You go to 'Posterize' and input a value of 8. Iirc, it will use RGB values of 0, 36, 72, 109, 145, 182, 218, and 255, iirc
I've never tried using posterize to clean up a low color count rgb image - only to reduce colors from high color count rgb images - so I can't confirm if that's how it behaves, but it was my understanding that it does clamp them but it just uses a nearist neighbor and clamps the colors down to a range defined in the value you input; That would be awesome if that's how it works though, each number representing a bit depth?
I looked into it briefly and it sounds like the number represents the # of colors per channel it will clamp the image down to. I am not certain it relates to bit depth, nor will set the colors to be any different value other than choosing the nearist neighboring color on the gamut