bump SUNDIALS submodule to v6.7.0 (blocked by CAMP not supporting SUNDIALS >= 6)

[slayoo]

[slayoo]

this fails with:

      /home/runner/work/PyPartMC/PyPartMC/gitmodules/camp/src/camp_solver.c:145:11: error: too few arguments to function ‘N_VNew_Serial’
        145 |   sd->y = N_VNew_Serial(n_dep_var * n_cells);
            |           ^~~~~~~~~~~~~

We need to update SUNDIALS calls within CAMP as it was done by @alhirzel for PartMC in https://github.com/compdyn/partmc/commit/db9d8dc74cce2e73fb0ae9c83e6df003e112b26f