Consider the block upper triangular matrix $M = \begin{pmatrix} A & B \\ 0 & C \end{pmatrix}$ where $A$ is $2 \times 2$, $C$ is $3 \times 3$, $B$ is $2 \times 3$, and $0$ is the zero block. If $A$ has eigenvalues $\{1, 2\}$ and $C$ has eigenvalues $\{3, 4, 5\}$, what are the eigenvalues of $M$?