Changelog entry for the move from key_ext to key_custom functions

Document psa_generate_key_ext() and psa_key_derivation_output_key_ext() as
deprecated in favor of psa_generate_key_custom() and
psa_key_derivation_output_key_custom(), and no longer declared in C++ builds.

Resolves Mbed-TLS#9020.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

ChangeLog.d/psa_generate_key_custom.txt

@@ -0,0 +1,14 @@
+API changes
+   * The experimental functions psa_generate_key_ext() and
+     psa_key_derivation_output_key_ext() are no longer declared when compiling
+     in C++. This resolves a build failure under C++ compilers that do not
+     support flexible array members (a C99 feature not adopted by C++).
+     Fixes #9020.
+
+New deprecations
+   * The experimental functions psa_generate_key_ext() and
+     psa_key_derivation_output_key_ext() are deprecated in favor of
+     psa_generate_key_custom() and psa_key_derivation_output_key_custom().
+     They have almost exactly the same interface, but the variable-length
+     data is passed in a separate parameter instead of a flexible array
+     member.