set_over in matplotlib is being deprecated

See, e.g., the nightly job here: https://github.com/scipp/plopp/actions/runs/26077705769/job/76672058232

```
PendingDeprecationWarning: The set_over function will be deprecated in a future version. Use cmap.with_extremes(over=...) or Colormap(over=...) instead.
```