rectifier capacitor 20V -> 50V

The rectifier capacitor had a spec of 20V which was correct when 12 VAC
was being rectified.

  Vp = Vrms*sqrt(2)
     = 12*sqrt(2)
     = 16.971

  16.971 * 1.10 = 18.668 < 20

However, the power supply was changed to use a 24 VAC input. And the
capacitor spec wasn't updated.

  Vp = Vrms*sqrt(2)
     = 24*sqrt(2)
     = 33.941

  33.941 * 1.10 = 37.335 > 20 !!!

Update the design to use a 50V capacitor which provides more than a
10% margin of safety.