matplotlib.cm.get_cmap is deprecated and will be removed in Matplotlib 3.11

src/graphing/heatmap.py:113 calls matplotlib.cm.get_cmap, which emits on every
heatmap plot:
MatplotlibDeprecationWarning: The get_cmap function was deprecated in Matplotlib 3.7
and will be removed in 3.11. Use ``matplotlib.colormaps[name]`` or
``matplotlib.colormaps.get_cmap()`` or ``pyplot.get_cmap()`` instead.
Reproduces on the current notebooks (cells calling plot_heatmaps and
plot_heatmaps_logarithmic in both GCGC.ipynb and FeatureExamples.ipynb). Both
heatmap functions will break outright once users upgrade to Matplotlib 3.11.

Start at src/graphing/heatmap.py:113 and inspect the plot_heatmaps and plot_heatmaps_logarithmic functions. Run the relevant cells in GCGC.ipynb and FeatureExamples.ipynb before and after the change; done means both heatmap functions work without the Matplotlib deprecation warning and remain compatible with Matplotlib 3.11.

Written by the indexing model from the issue text.