bug#63231: 28.1; image-converter should remove warning messages while conversion

[Phil Dyer via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

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I have hit this bug while opening heic images on Linux.

Imagemagick v7 is deprecating the "convert" CLI. It issues a warning:

'''
WARNING: The convert command is deprecated in IMv7, use "magick" instead of "convert" or "magick convert"
'''

It still produces the PNG, however, the resulting PNG has the following header:

'''
WARNING: The convert command is deprecated in IMv7, use "magick" instead of "convert" or "magick convert"

\3fff89PNG
\^Z
\^@\^@\^@
IHDR\^@\^@\3fff8c\^@\^@\^R\^P\^H\^B\^@\^@\^@p\3fffd3\3fffc0\3fffb1\^@\^@\^@ cHRM\^@\^@z&\^@\^@\3fff80\3fff84\^@\^@\3ffffa\^@\^@\^@\3fff80\3fffe8\^@\^@u0\^@\^@\3fffea`\^@\^@:\3fff98\^@\^@\^Wp\3fff9c\3fffbaQ<\^@\^@\^@\^FbKGD\^@\3fffff\^@\3fffff\^@\3fffff\3fffa0\3fffbd\3fffa7\3fff93\^@\^@\^@ pHYs\^@\^@\^K\^R\^@\^@\^K\^R\^A\3fffd2\3fffdd~\3ffffc\^@\^@\^@\^AorNT\^A\3fffcf\3fffa2w\3fff9a\^@\^@\3fff80\^@IDATx\3fffda\3fffc4\3ffffd\3fffe9\3fff97d\3fffd9q'\3fff88\3fff99\3fffdd\3ffff5-\3fffbe{\3fffec\3fff91\3fff99\3fff91k\3fffad@\^U
....more png content below
'''

A valid PNG should not have a CLI warning embedded in it. Both warnings and errors should be placed into the messages/warnings/error buffers or similar, not placed inside the string representation of the image file, which I can't read anyway without calling (image-convert) directly. What currently happens when opening a .hiec file with `image-use-external-converter` set to t​ is that the entire PNG string is dumped into the messages buffer and the original image buffer displays a blank square. Since `*message*​​` truncates older lines, the warning message and reason for failure is not visible anyway.

Properly handling the imagemagick v7 cli is important, but is a separate issue. The warning should not be returned inside the string representation of the converted image.

Regards,
Phil

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