Bump baseline to SciPy 1.18

Tasks

Read Release Notes.
Fix code and CI to support the new version.
- FlexCI: Update .pfnci/schema.yaml and .pfnci/matirx.yaml.
- Update Requirements: For NumPy/SciPy, bump the pinned minimum/maximum version in pyproject.toml (see [info] NumPy/SciPy new version pinning recommendation #4773.)
Fix documentation.
- Update Requirements section in the Installation Guide (note: the baseline version must be updated only in the development branch).
- Update docs/requirements.txt to use the latest versions.
Backport the above PR.
Apply fixes needed to follow the specification changes.

An additional reason to support SciPy 1.18 is that it is required for Python 3.15 testing with SciPy.

Possible smaller follow-ups that I don't think can happen immediately (or shouldn't be bunched up):

SciPy sparse is transitioning triu/tril to return sparse arrays and giving a warning. We could warn, but it feels like this may be early for us.
For sparse linalg, matvec and rmatvec have a warning to deprecate on column vectors with a FutureWarning. (However, this looks like it is meant to be DeprecationWarning that is just more visible, so I don't think this is pressing.)

Let me re-open. This is done for now, but when we bump baseline there are some things to check/follow-up on.