Add support for free-threading wheels

[imciner2]

This enables the QDLDL wheels to be built with the free-threading support (aka. no-GIL).

I think all we need to do is add the py::mod_gil_not_used() to our module, and that will also disable all our other calls releasing/acquiring the GIL.

I am basing the cibuildwheel change off of the documentation here: https://py-free-threading.github.io/ci/#building-free-threaded-wheels-with-cibuildwheel

[imciner2]

To get this to work I needed to update the cibuildwheel version to the latest, and also remove our use of the manylinux2014 container. Once I did that, we now use pre-built versions of our dependencies (i.e., numpy) on the newer Python versions, and we also support Python 3.14 with this.