The good news is the cavity light is a replaceable part. The part can be ordered from the
website. The part number is WB36X10131. It is not very expensive.
Now the bad news:
To change this cavity light requires removing the outer cover of the microwave and this exposes some very dangerous parts. A technician must complete this type of repair.
Microwaves are inherently dangerous to service. Internal capacitors can retain a lethal electrical charge even when the microwave is unplugged. A microwave radiation leakage test must be performed on the unit following any repair. I recommend having a technician install the light bulb.