Simultaneous Trapping of C<sub>2</sub>H<sub>2</sub> and C<sub>2</sub>H<sub>6</sub> from a Ternary Mixture of C<sub>2</sub>H<sub>2</sub>/C<sub>2</sub>H<sub>4</sub>/C<sub>2</sub>H<sub>6</sub> in a Robust Metal–Organic Framework for the Purification of C<sub>2</sub>H<sub>4</sub>