docs: add missing deprecation

Author: kenjis

user_guide_src/source/changelogs/v4.2.2.rst

@@ -32,6 +32,7 @@ Deprecations
 
 - The parameters of ``Services::request()`` are deprecated.
 - The first parameter ``$cacheConfig`` of ``CodeIgniter::gatherOutput()`` is deprecated.
+- The second parameter ``$ifNotExists`` of ``Forge::_createTable()`` is deprecated.
 
 Bugs Fixed
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