Is it necessary to separate ground planes in a PCB if there are different voltages?
No. There are very few instances in which separate grounds are necessary or good. In this case, you don't even have a mixed-signal system. There's no reason to have split ground planes, and separate grounds will only make things worse.

Take care to manage your return currents and keep the buck/boost regulator layout compact. Everything else appears to be low-frequency digital or power, so there shouldn't be any signal integrity weirdness.