devel/libzim: Update 8.2.1 → 9.2.1

Selected list of improvements:

• getMediaCount() does not fail anymore if M/Counter is missing
• Reintroduce optimization of Entry::getItem()
• C++17 compatible code
• Bump-up ZIM format minor version to 6.1.2
• Optimize checksum calculation
• Better handling of split ZIM files

See also:
https://github.com/openzim/libzim/releases/tag/9.0.0
https://github.com/openzim/libzim/releases/tag/9.1.0
https://github.com/openzim/libzim/releases/tag/9.2.0
https://github.com/openzim/libzim/releases/tag/9.2.1

This is a maintainer update (olce.freebsd.ports@certner.fr).

Approved by: jrm (ports)
Approved by: emaste (mentor)