STM32 supports lower CPU clock speed.
I think modifying RCC_CFGR changes clock.
But some numbers calculated by 120MHz (defined as CPU_CLOCK_HZ).
So please change clock speed carefully.
For detail of RCC registers, please refer ST information.
You can run as slow as you want. But you need to calculate all the PLL settings and update the platform_config.h
Use the STM clock calculation tool to generate the values
We haven't tested extensively at low frequencies but 48MHz should work