Fix number of pixels in A1 paper preset

A1 300ppi: I'm not sure where did "7008" come from, every source on the Internet have uses 7016, including GIMP's template https://gitlab.gnome.org/GNOME/gimp/-/blob/master/etc/templaterc

A1 600ppi: The number is just straight wrong, perhaps a copy-pasting mistake. I replaced it with a number that I think is the most accurate and cross referenced a few sources including https://www.a1-size.com/a1-size-in-pixels/