Reduce 8.4 volts to 6 volts
Use this simple circuit to lift the voltage level to that of the servo:







simulate this circuit &ndash; Schematic created using CircuitLab

What does it do? Q2 switches the output to the servo between 0V and +8.4V (or whatever you apply there). It's normally conducting, because of the current into its base through R2, so the servo output is 0V. If the RPi GPIO is switched ON, Q1 conducts, so Q2 falls short of base current and stops conducting. Then, the servo output is pulled to +8.4V through R3.