doc: releases: add a note about the kscan removal

Add a note about the removal of the kscan subsystem.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>

Author: fabiobaltieri

doc/releases/release-notes-4.2.rst

@@ -58,6 +58,8 @@ Removed APIs and options
 * Removed the ``--disable-unrecognized-section-test`` Twister option. Test has been removed and the
   option became the default behavior.
 
+* Removed the deprecated ``kscan`` subsystem.
+
 Deprecated APIs and options
 ===========================