libcupsfilters: Added gray PCLm support to ghostscript()

libcupsfilters: Added gray PCLm support to ghostscript()

PCLm does not only support 24-bit sRGB bur also 8-bit sGray. All PCLm
printers, regardless of whether they are color or monochrome accept
these two color spaces. Monochrome printers have to convert color into
gray if needed.

Ghhostscript has a grayscale PCLm output device, "pclm8" since version
9.56.0. We now make use of it if the job is monochrome. This also
allows us to supply the cortrecr sGray ICC profile instead of the sRGB
one.