graphics/libheif: update to 1.6.0

• Update MAINTAINER address
• Fix stage-qa warning
• Drop FreeBSD 10 fix after r483654

Changes: https://github.com/strukturag/libheif/releases
Changes: https://github.com/strukturag/libheif/compare/v1.3.2...v1.6.0
PR: 241607 241900
Submitted by: Sebastian Steinmetz (maintainer)
Approved by: maintainer timeout (2 weeks, old email)