gh-36365: Libgap cubegroup
This is refreshing the modified file
- use libgap instead of gap
- use unicode for display
- a few other small details
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.
URL: https://github.com/sagemath/sage/pull/36365
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert
