Solver_interface: Soon deprecated Eigen API

[afabri]

Issue Details

In the Surface Mesh Deformation testsuite using the master branch of Eigen we get an error.

The error does not show up in the Solver_interface testsuite, which means that it is not complete.

This commit on the Eigen gitlab gives a hint what we have to change.

Environment

• Other libraries versions if used (Eigen, TBB, etc.): Eigen master branch

[lrineau]

See also the corresponding PR https://gitlab.com/libeigen/eigen/-/merge_requests/857 and the discussions in it.

[lrineau]

@soesau This issue is exactly the same as in https://cgal.geometryfactory.com/CGAL/testsuite/CGAL-5.5-I-69/Surface_mesh_deformation/TestReport_lrineau_Ubuntu-GCC_master_CXX20-Release.gz

[MaelRL]

Fixed by https://github.com/CGAL/cgal/pull/6562.