Re-rename builtin seaborn styles to not include a dot.

... as we may want to use dots later to support third-party styles, e.g.
`foo.bar` could be used to mean `foo/bar.mplstyle` where `foo` is a
regularly installed python package.

Author: anntzer

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

@@ -3,6 +3,6 @@ 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
+0.9.  To prevent confusion, the style files have been renamed "seaborn-v0_8",
+"seaborn-v0_8-bright", "seaborn-v0_8-colorblind", etc.  Users are encouraged to
 directly use seaborn to access the up-to-date styles.