Use sparse Cholesky decomposition with ldiv!

[JoshuaLampert]

Currently, we use an LU factorization for the BBMBBMEquations1D and BBMBBMVariableEquations1D although the matrix is spd, which means we could use a Cholesky decomposition. We did this because ldiv! was not supported for sparse Cholesky factorizations, see the discussion in #108 (comment). For the SvaerdKalischEquations1D we use a sparse Cholesky decomposition, but use \ instead of ldiv!, again, because ldiv! was not implemented for sparse Cholesky decompositions.
Once JuliaSparse/SparseArrays.jl#547 (thanks for the PR @ranocha!) is available, it should be possible to use a sparse Cholesky decomposition together with ldiv! for all equations. This will be with Julia v1.12.

[JoshuaLampert]

In this case, we could reuse the function solve_system_matrix!, which is currently only used for the Serre-Green-Naghdi equations for the others, too.