changelog: add note for mbedtls_x509write_crt_set_serial() deprecation

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/ChangeLog.d/9892.txt b/ChangeLog.d/9892.txt
new file mode 100644
index 0000000..01d21b6
--- /dev/null
+++ b/ChangeLog.d/9892.txt
@@ -0,0 +1,4 @@
+Removals
+   * Remove deprecated mbedtls_x509write_crt_set_serial(). The function was
+     already deprecated and superseeded by
+     mbedtls_x509write_crt_set_serial_raw().