Kycon_KDMIX-SL1-NS-WS-B15 - minor fixes

• Retract silkscreen from nominal board edge
• Adjust round rect radius -> 0.25mm
• Rotate to pin 1 at top left
• Add pin 1 marker visible after assembly
• Kicad 6 -> 7 in model path
• Fab layer pin 1 marker (chevron pointing at pin 1)

I also zeroed the model offset, but I assume that was set to match the manufacturer STEP. So not sure about if that's a good idea.

Original Github PR: https://github.com/KiCad/kicad-footprints/pull/1651

KLC check: annulus size 0.125mm, not changed (justification in the original PR).