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.