Changed quantize default dither to FLOYDSTEINBERG

[radarhere]

https://pillow.readthedocs.io/en/stable/reference/Image.html#PIL.Image.Image.quantize

dither – Dithering method, used when converting from mode “RGB” to “P” or from “RGB” or “L” to “1”. Available methods are NONE or FLOYDSTEINBERG (default). Default: 1 (legacy setting)

It's weird that the documentation specified twice what the default was, but I guess that the FLOYDSTEINBERG constant has a raw value of 1. Oh wait, no, it doesn't.

Pillow/src/PIL/Image.py

Lines 168 to 172 in 29b7030

	# dithers
	NEAREST = NONE = 0
	ORDERED = 1 # Not yet implemented
	RASTERIZE = 2 # Not yet implemented
	FLOYDSTEINBERG = 3 # default

If you search for 'dither' in Image.py, _imaging.c and Convert.c, it makes no difference in the code whether it is 1 or 3.

So this PR suggests changing the quantize() default dither to FLOYDSTEINBERG. It should have no effect on the behaviour of Pillow, but it will stop the docstring from being confusing.

While I'm here, I also stop passing dither to the C function tobilevel, since it is unused.