Preparation,Absorption Spectra,and Luminescence Properties of Er<sub>2sub>O<sub>3sub>- and Yb<sub>2sub>O<sub>3sub>-Doped Oxyfluoride Glasses in the CaF<sub>2sub>–SiO<sub>2sub>–B<sub>2sub>O<sub>3sub>–Bi<sub>2sub>O<sub>3sub>–TiO<sub>2sub>–ZnO–Y<sub>2sub>O<sub>3sub> System