Why am I reading low AC voltage on my garage opener light?
There are two possibilities that I can think of that may be causing the problem with the light on your garage door opener. 


There may be an internal relay within the opener that turns on and off to control the light. The electrical contacts in the relay may have become burned and producing such a high resistance connection that even a tiny load drops the voltage way down. In this case it may be possible for someone reasonably handy with a soldering iron to remove and replace the relay.
In the second case the light bulb socket on the opener may be controlled via an AC switch component called a triac. These switch on and off via the presence of a electrical signal from the opener control circuit board. Triac switches can fail and cause the symptom you see where just a fraction of the applied voltage makes it to the light bulb socket. Failure of these is most commonly caused by power line spikes from storms and lightning. They can also fail due to current surges caused when a light bulb burns out and the fractured filament swings around and momentarily shorts out the internal light bulb electrodes. Triac replacement is possible to do for someone experienced in working with printed circuit boards. The tricky part can be identifying the proper replacement component.