The minimal polynomial of a matrix $A$ is the monic polynomial of lowest degree such that $p(A) = 0$. It divides the characteristic polynomial. For the upper triangular matrix $A = \begin{pmatrix} 2 & 1 & 0 \\ 0 & 2 & 0 \\ 0 & 0 & 3 \end{pmatrix}$, what is the minimal polynomial?