Default animation.convert_args to ["-layers", "OptimizePlus"].

On the simple_anim.py example, adding e.g.
```
plt.rcParams["animation.writer"] = "imagemagick"
plt.rcParams["animation.convert_args"] = []
ani.save("old.gif")
plt.rcParams["animation.convert_args"] = ["-layers", "OptimizePlus"]
ani.save("new.gif")
```
shows that OptimizePlus yields a >2.5x decrease in gif size.

Author: anntzer

doc/api/next_api_changes/behavior/23371-AL.rst

@@ -0,0 +1,4 @@
+The default ``rcParams["animation.convert_args"]`` changed
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+It now defaults to ``["-layers", "OptimizePlus"]`` to try to generate smaller
+GIFs.  Set it back to an empty list to recover the previous behavior.

lib/matplotlib/mpl-data/matplotlibrc

@@ -779,7 +779,7 @@
 ## ImageMagick in the registry (as convert is also the name of a system tool).
 #animation.convert_path: convert
 ## Additional arguments to pass to convert.
-#animation.convert_args:
+#animation.convert_args: -layers, OptimizePlus
 #
 #animation.embed_limit:  20.0     # Limit, in MB, of size of base64 encoded
                                   # animation in HTML (i.e. IPython notebook)