Bump version to 2.28.9

Soversions unchanged.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/doxygen/mbedtls.doxyfile b/doxygen/mbedtls.doxyfile
index 971a8b2..b75597d 100644
--- a/doxygen/mbedtls.doxyfile
+++ b/doxygen/mbedtls.doxyfile
@@ -1,4 +1,4 @@
-PROJECT_NAME           = "Mbed TLS v2.28.8"
+PROJECT_NAME           = "Mbed TLS v2.28.9"
 OUTPUT_DIRECTORY       = ../apidoc/
 FULL_PATH_NAMES        = NO
 OPTIMIZE_OUTPUT_FOR_C  = YES