A water reservoir is shaped like an inverted cone with height $H=12$ m and top radius $R=4$ m. Water is being pumped into the reservoir at a constant rate of $2\pi$ m$^3$/min. How fast is the water level rising ($dh/dt$) when the depth $h$ is $6$ m? (Volume of a cone $V = \frac{1}{3}\pi r^2 h$)