MNT: Use warning instead of info log level

Closes matplotlib#27073.

For details see https://github
.com/matplotlib/issues/23422#issuecomment-2363859957.

Author: timhoffm

lib/matplotlib/category.py

@@ -221,10 +221,10 @@ def update(self, data):
             if val not in self._mapping:
                 self._mapping[val] = next(self._counter)
         if data.size and convertible:
-            _log.info('Using categorical units to plot a list of strings '
-                      'that are all parsable as floats or dates. If these '
-                      'strings should be plotted as numbers, cast to the '
-                      'appropriate data type before plotting.')
+            _log.warning('Using categorical units to plot a list of strings '
+                         'that are all parsable as floats or dates. If these '
+                         'strings should be plotted as numbers, cast to the '
+                         'appropriate data type before plotting.')
 
 
 # Register the converter with Matplotlib's unit framework