[ios_platform_images] Switch to loadImage

[stuartmorgan]

Replaces the deprecated loadBuffer override with the newer loadImage, removing the need to use the deprecated DecoderBufferCallback class.

This is largely a no-op because the difference between DecoderBufferCallback and ImageDecoderCallback is entirely in optional parameters the package doesn't use.

Fixes flutter/flutter#143880

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[flutter-dashboard]

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[stuartmorgan]

test-exempt: code refactor with no semantic change