Drop pybind11 submodule and use it as a build-time dependency

[agriyakhetarpal]

While both pathways are fine, we may find it easier to use the pybind11 Python package instead of using it as a submodule.

This ports one of my older PRs; pybamm-team/PyBaMM#3560 – which was stalled due to other reasons when the IDAKLU solver was considered an optional requirement.

[agriyakhetarpal]

The builds are passing, but somehow we're picking up a different Python version at build time rather than the one requested in the matrix (a job with 3.12 is still building wheels for 3.13, which means that the idaklu shared object is not found as a result). It's strange!

[agriyakhetarpal]

Setting PYBIND11_FINDPYTHON to OFF so as to not let it change the Python interpreter midway results in passing builds, but builds with cibuildwheel fail. :D