docs: add index.php/spark changes in changelog

Author: kenjis

user_guide_src/source/changelogs/v4.2.0.rst

@@ -13,6 +13,9 @@ BREAKING
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 - The method signature of ``Validation::setRule()`` has been changed. The ``string`` typehint on the ``$rules`` parameter was removed. Extending classes should likewise remove the parameter so as not to break LSP.
+- The ``CodeIgniter\CodeIgniter`` class has a new property ``$context`` and it must have the correct context at runtime. So the following files have been changed:
+    - ``public/index.php``
+    - ``spark``
 
 Enhancements
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