games/dMagnetic: Update to 0.31
From ChangeLog: https://www.dettus.net/dMagnetic
- New input formats: NIB and 2MG
- Improved Sixel mode
- New -dumpmag and -dumpgfx command line parameters
- Moved the manpage to section 6 (games)
PR: 256798
Reported by: dettus@dettus.net