Update `plot_heatmap()` to be a table instead of a plot

[nikosbosse]

I believe I mentioned it in the past but I am still uncomfortable with the plot_heatmap() function. The output here should be a table, not a plot. This comes with accessibility issues, as the values are no longer accessible to screen readers, and it’s harder to copy-paste the values. There are good options to have tables with coloured cells today, and it should probably be the way to go.

Originally posted by @Bisaloo in https://github.com/epiforecasts/scoringutils/pull/791#pullrequestreview-2003401135