[DOCS] Add migration note for removed GFX/gdlib setting

Resolves: #102113
Releases: main
Change-Id: I7304567faee9d0883d3c24d93b4b133fba7852d0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81442


Reviewed-by: Chris Müller <typo3@krue.ml>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Chris Müller <typo3@krue.ml>

typo3/sysext/core/Documentation/Changelog/13.0/Breaking-102113-RemovedLegacySettingGFXgdlib.rst

@@ -29,6 +29,10 @@ are found.
 Migration
 =========
 
-The configuration value has been removed without replacement. No migration is neccesary.
+The configuration value has been removed without replacement.
+
+Custom code that relied on :php:`$GLOBALS['TYPO3_CONF_VARS']['GFX']['gdlib']`
+should instead also adopt to the simpler check
+ :php:`if (class_exists(\GdImage::class))`.
 
 .. index:: LocalConfiguration, FullyScanned, ext:core