math/pantheon-calculator: Update to 1.6.0

• Add submitter back as maintainer
• Fix deprecated function

Changelog: https://github.com/elementary/calculator/releases/tag/1.6.0

PR: 252167
Submitted by: Olivier Duchateau <duchateau.olivier@gmail.com> (renewed maintainer)
Approved by: arrowd (mentor)
Differential Revision: https://reviews.freebsd.org/D27780