Solver_interface: unknown group in documentation

[albert-github]

Analogous to the issues #7691, #7692 and #7693 adding a warning to doxygen gives me the warning

Solver_interface/include/CGAL/Eigen_sparse_matrix.h:316: warning: Found non-existing group 'PkgSolverInterfaceRefLS' for the command '@ingroup', ignoring command

• where does this \ingroup belong to or should the line be removed?

[afabri]

Do you have the same for this \ingroup ?

And we have this \defgroup so it should work.

[albert-github]

I'm just writing an overview in #7791 with some observations

[afabri]

In fact here we had one Ref too much in a label. Fixed by #7804

[albert-github]

This issue is only relevant when using the doxygen master version and as the CGAL versions 5.5. and 5.6 don't use this version in the distribution it is not relevant here.
With the current CGAL master and using the doxygen master version the warnings are gone based on the fix #7804.

I think the issue can be closed