doc: migration-guide: add note for changes in UpdateHub

Add a note about the removal of legacy Mbed TLS crypto support from
UpdateHub.

Signed-off-by: Valerio Setti <[email protected]>

Author: valeriosetti

doc/releases/migration-guide-4.3.rst

@@ -349,6 +349,13 @@ Shell
   compatibility.
   (:github:`92677`).
 
+UpdateHub
+=========
+
+* Only PSA Crypto API are used for crypto support and legacy Mbed TLS crypto is removed.
+  :kconfig:option:`CONFIG_UPDATEHUB` will automatically enable Mbed TLS PSA Crypto implemementation
+  if TF-M is not enabled in the build.
+
 .. zephyr-keep-sorted-stop
 
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