Impact of high‐temperature sintering on SiO2−LiO2−Al2O3−K2O−P2O5${\rm SiO}_2{-}{\rm LiO}_2{-}{\rm Al}_2{\rm O}_3{-}{\rm K}_2{\rm O}{-}{\rm P}_2{\rm O}_5$ glass ceramics with ZrO2${\rm ZrO}_2$ and Y2O3${\rm Y}_2{\rm O}_3$ additives