LTC2311 CMOS SPI connection with STM32
CNV pin does not need precsion to achieve maximum rate, but it needs precision to achieve minimum jitter, i.e. to sample at precice intervals.

It depends on what your precision needs are, but just to get you started, use a GPIO pin. It can be a GPIO pin or the Chip Select pin, but it depends on if you use hardware or software controlled Chip Select to begin with. The STM32 does not care about Chip Select state when it is master. If you want precision, to sample at exactly uniform intervals, perhaps you would like to use a timer output channel pin so you can use a hardware timer for accurate sampling intervals.

If your STM32 MCU is running at 48 MHz, most likely highest clock you can use with the SPI peripheral is 24 MHz. So it will limit your bit transfer speed. However, you did not mention which STM32 so you better check yourself what clock frequencies it can use, and which SPI frequencies are possible with that clock frequency.