Resolves #8007

https://pillow.readthedocs.io/en/stable/reference/ImageCms.html#PIL.ImageCms.buildTransform and https://pillow.readthedocs.io/en/stable/reference/ImageCms.html#PIL.ImageCms.buildProofTransform describe inMode and outMode as

String, as a valid PIL mode that the appropriate profile also supports

However, "RGBA;16B", "L;16", "L;16B", "YCCA" and "YCC" are modes that can be interpreted, and there is a default case if the mode is not recognised.

This deprecates modes that are not Pillow modes, and also deprecates the default value if the mode is not able to be mapped. I've also added I;16, I;16L, I;16B, YCbCr as possible replacement values.