Link checker fails on Python 3.12

[AlexanderWells-diamond]

As of Python3.12, the numpy package numpy.distutils has been deprecated. This causes a problem in the link check workflow. Here's an example failed workflow.

The skeleton should be modified so that its linkchecker works. The simplest solution for now is likely pinning the python version in linkcheck.yml to specify a Python version <=3.11.