Use oldest-supported-numpy

I just noticed
https://github.com/dwavesystems/dwave-preprocessing/blob/be12b72b1e9c8ef450f83e93b844801df641e7bd/pyproject.toml#L6

This should almost always be oldest-supported-numpy if it's in a pyproject.toml file. https://github.com/scipy/oldest-supported-numpy - see https://scikit-hep.org/developer/packaging#special-additions-numpy or the note at https://cibuildwheel.readthedocs.io/en/stable/options/#before-build for example.

Inspect the referenced pyproject.toml dependency at the linked line and compare its NumPy requirement with the oldest-supported-numpy guidance cited in the issue. Confirm the packaging metadata uses the recommended dependency, then verify the resulting project configuration still parses correctly.

Written by the indexing model from the issue text.