It’s very likely that the valves will start just fine with less than 22VAC. As I explained in this blog post:
http://rayshobby.net/understanding-24vac-sprinkler-valves/
the solenoid valves typically require a high impulse voltage to get energized. Once energized, the voltage can go much lower and that’s enough to keep it energized.
The firmware will not be able to adjust the output voltage — this completely depends on the output voltage from your transformer, and the firmware has no control over it.