Revaluate if backend should require tiffs to be in RGB/RGBA format before converting to PNG

[yousefmoazzam]

There's only a need for grayscale images in the frontend, so this conversion to RGB/RGBA is possibly unnecessary: https://github.com/DiamondLightSource/TomoHub/blob/f050ce39026700664bf4f194ee429aa61289cbfd/backend/routers/proxy.py#L146-L152