Redundant version requirement

[mglisse]

https://github.com/GUDHI/gudhi-devel/blob/f44339e0fb3d3f622971b8356c71b9a6aea72a58/src/common/doc/main_page.md?plain=1#L158

We have a global requirement that CGAL has to be >= 5.1 (because of cmake). And we repeat that requirement for each package that uses CGAL.

• it looks redundant
• the code would actually work fine with older versions of CGAL, it is only the cmake scripts used to build our examples/tests/plugin that fail

So I think I would prefer not to specify the version in places where it matches the global one.