Bump version to 2.28.9

Soversions unchanged.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/doxygen/input/doc_mainpage.h b/doxygen/input/doc_mainpage.h
index 7a24021..d7c6463 100644
--- a/doxygen/input/doc_mainpage.h
+++ b/doxygen/input/doc_mainpage.h
@@ -10,7 +10,7 @@
  */
 
 /**
- * @mainpage Mbed TLS v2.28.8 API Documentation
+ * @mainpage Mbed TLS v2.28.9 API Documentation
  *
  * This documentation describes the internal structure of Mbed TLS.  It was
  * automatically generated from specially formatted comment blocks in