Change C1021 to 3pF for Rev 5.3

[AleksaBjelogrlic]

As recommended in #410 (comment), change C1021 to a 4pF part from the existing 5pF, no need for the 200V rating, 50V is fine since this is on the bottom leg of the attenuator - part number is KEMET CBR04C409B5GAC

Rev 5.2 attenuator schematic below for context:

[AleksaBjelogrlic]

Done in 3ab6026, attenuator circuit looks like this now:

[AleksaBjelogrlic]

Reopening this issue based off of new testing in #410 (comment), due to the #416 issue.

C1021 should be changed to 3pF instead of 4pF, I will change this ECO to reflect that