graphics/ImageMagick6: upgrade to 6.9.12-12

Changelog at https://legacy.imagemagick.org/script/changelog.php.

Also change WWW: to the legacy version.

Note: this might fix some vulnerabilities, e.g. CVE-2021-20244,
CVE-2021-20243, CVE-2021-20176 or CVE-2020-27829, but this is not clear
for me.

PR: 255818
Approved by: maintainer’s time-out