Changelog entry for PSA CBC-PKCS7 padding oracle fix

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
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+Security
+   * Fix a timing side channel in CBC-PKCS7 decryption that could
+     allow an attacker who can submit chosen ciphertexts to recover
+     some plaintexts through a timing-based padding oracle attack.
+     Credits to Beat Heeb from Oberon microsystems AG. CVE-TODO