Update version to 1.3.21
diff --git a/doxygen/input/doc_mainpage.h b/doxygen/input/doc_mainpage.h
index e49a383..3452e24 100644
--- a/doxygen/input/doc_mainpage.h
+++ b/doxygen/input/doc_mainpage.h
@@ -4,7 +4,7 @@
  */
 
 /**
- * @mainpage mbed TLS v1.3.20 source code documentation
+ * @mainpage mbed TLS v1.3.21 source code documentation
  *
  * This documentation describes the internal structure of mbed TLS.  It was
  * automatically generated from specially formatted comment blocks in