Deprecate saving I mode images as PNG

[radarhere]

Resolves #9022

PNG images can only have a maximum of 16 bits per channel - https://www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html

Bit depth is a single-byte integer giving the number of bits per sample or per palette index (not per pixel). Valid values are 1, 2, 4, 8, and 16, although not all values are allowed for all color types.

However, Pillow still allows saving PNG images as I mode, by clipping the data.

The issue has requested that we prevent this automatic transformation, so this PR deprecates it.

[hugovk]

Can we give advice on what people should do instead?

[radarhere]

I've updated the commit to append

Instead, the image can be converted to another mode before saving::

    from PIL import Image
    im = Image.new("I", (1, 1))
    im.convert("I;16").save("out.png")

[radarhere]

I noticed that the deprecation warning wasn't being emitted for

from PIL import Image
im = Image.new("I", (1, 1))
im.save("out.png")

This is because the call to deprecate() isn't in the function that the user calls (im.save()), it's one level deeper than that (PngImagePlugin._save). I've added a commit to pass a custom stacklevel to deprecate().