games/dMagnetic: Update to 0.21

• Update to 0.21
• Remove EXTRACT_SUFX and use USE=tar:bz2 instead (pet portlint)

Changelog:

http://freshcode.club/projects/dmagnetic
http://www.dettus.net/dMagnetic/old_releases.html

PR: 243379
Reported by: Thomas Dettbarn (maintainer)
Reviewed by: koobs
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D23214