Rename outdated seaborn styles.

They are kept available under a versioned name for backcompat.

Author: anntzer

doc/api/next_api_changes/deprecations/22317-AL.rst

@@ -0,0 +1,8 @@
+seaborn styles renamed
+~~~~~~~~~~~~~~~~~~~~~~
+Matplotlib currently ships many style files inspired from the seaborn
+library ("seaborn", "seaborn-bright", "seaborn-colorblind", etc.) but they
+have gone out of sync with the library itself since the release of seaborn
+0.9.  To prevent confusion, the style files have been renamed "seaborn0.8",
+"seaborn0.8-bright", "seaborn0.8-colorblind", etc.  Users are encouraged to
+directly use seaborn to access the up-to-date styles.