Use imageflip() in WP_Image_Editor_GD::flip()

[arshidkv12]

WP_Image_Editor_GD::flip() uses negative dimensions with imagecopyresampled().

With PHP 8.6's bundled GD 2.4.0, this causes Tests_Image_Editor_GD::test_flip to fail:

Failed asserting that 0 is identical to 8434407

The same operation works with PHP 8.5/GD 2.3.3. imageflip() produces the expected result on both versions.

This patch replaces the imagecopyresampled() implementation with imageflip() while preserving the existing flip behavior.

Use imageflip() in WP_Image_Editor_GD::flip() instead of imagecopyresampled() with negative dimensions.

This fixes image flipping with PHP 8.6 / GD 2.4.0, where the existing implementation can produce incorrect pixel data.

[Trac ticket: https://core.trac.wordpress.org/ticket/66113
](https://core.trac.wordpress.org/ticket/66113)

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