* This extends PR #47285 to properly include some of the required version constrains of cgal 6 incl C++ standard. It also adds the new no-gmp backend as a variant.
* fix style
* disable cgal@6 +demo variant as the demos require qt6 which is not in spack
* disable the gmp variant until clarity on how its supposed to work is provided. bound shared and header_only variants to relevant versions
* Fix missing msvc compiler limit, fix variant left in
* Add more comments. Better describe the gmp variant. Remove testing code
* fix style
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