[!!!][TASK] Remove legacy setting 'GFX/gdlib_png'

Another GFX-setting that enables PNG generation for temporary
files. PNG files offer many benefits, one of them being faster
generation using image processors.

We previously held back on removing this setting because the
temporary images using gdlib_png were significantly darker than
the GIF files, but this stems from the TYPO3 default configuration
using the RGB colorspace (if nothing else is set) in combination
with ImageMagick. The sRGB colorspace, as recommended in the preset,
should be used.

If GraphicsMagick is used, generating temporary images using the RGB
colorspace works fine.

With this knowledge it is safe to remove the setting, but a migration
notice has been left notifying users of each image processor of
required migration steps.

Resolves: #102020
Releases: main
Change-Id: I4be795406cbb010d4b668a2de588c4591c443cf3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81184


Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>