The current placeholder text color on captions is using a desaturated gray color – which is different from the other instances (paragraph/heading/list placeholder text). Let's update that to align w/ the other instances, which I believe is $gray.

(This issue applies to both Android and iOS, IIRC)

Note: we'll be updating to a new color palette in the near future, but we don't yet have that work slated, so let's do this in the meantime.

Related: #549