feat: replaced seaborn with matplotlib for correlation_heatmap (#850)

Closes #800 

### Summary of Changes

Removed seaborn with matplotlib for correlation_heatmap.
Matplotlib returns a slightly different plot, so we also replaced the
snapshot image.

But seaborn still remains as a dependency, because box_plots is still
using seaborn, see issue #849 .

---------

Co-authored-by: Lars Reimann <[email protected]>
Co-authored-by: megalinter-bot <[email protected]>

src/safeds/data/tabular/plotting/_table_plotter.py

@@ -97,7 +97,7 @@ def correlation_heatmap(self) -> Image:
         # TODO: implement using matplotlib and polars
         #  https://stackoverflow.com/questions/33282368/plotting-a-2d-heatmap
         import matplotlib.pyplot as plt
-        import seaborn as sns
+        import numpy as np
 
         only_numerical = self._table.remove_non_numeric_columns()._data_frame.fill_null(0)
 
@@ -115,15 +115,18 @@ def correlation_heatmap(self) -> Image:
                     " automatically expanding."
                 ),
             )
-            fig = plt.figure()
-            sns.heatmap(
-                data=only_numerical.corr().to_numpy(),
+
+            fig, ax = plt.subplots()
+            heatmap = plt.imshow(
+                only_numerical.corr().to_numpy(),
                 vmin=-1,
                 vmax=1,
-                xticklabels=only_numerical.columns,
-                yticklabels=only_numerical.columns,
-                cmap="vlag",
+                cmap="coolwarm",
             )
+            ax.set_xticks(np.arange(len(only_numerical.columns)), labels=only_numerical.columns)
+            ax.set_yticks(np.arange(len(only_numerical.columns)), labels=only_numerical.columns)
+            fig.colorbar(heatmap)
+
             plt.tight_layout()
 
         return _figure_to_image(fig)