You technically can heat milk in a stovetop kettle, but you'll need to be more careful about it.
For one, milk overflows more readily than water. That's because the fat that milk contains floats to the top, leaving the water to boil.
Then, the steam from the water gets trapped in the milk fat, which causes frothing.
You'll also need to pay more attention, so you don't scald or burn the milk.
Keeping all that in mind, I'd still highly recommend you just do it in a saucepan.