deskutils/freeplane: Update to 1.7.6

Changelog:

• Bugfix release

https://github.com/freeplane/freeplane/compare/release-1.7.5...release-1.7.6

PR: 236296
Submitted by: lbartoletti <lbartoletti@tuxfamily.org> (maintainer)
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D19519