What should be the voltage rating of bootstrap capacitor?
The boot-strap capacitor is switched from the output to either being at GVDD or GND.

In the datasheet, GVDD is specified to be 7V, but the maximum output is 14.4V.  So 16V is probably they lowest they could recommend, though it probably also needs to be a larger case size part to not lose to much capacity to Vbias if ceramic.

Halfway through this page there is a nice drawing of a bootstrap NMOS H-Bridge.