Bump version to 2.28.0

Executed ./scripts/bump_version.sh --version 2.28.0 --so-tls 14

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/doxygen/input/doc_mainpage.h b/doxygen/input/doc_mainpage.h
index 4ff63ce..5a0f3ec 100644
--- a/doxygen/input/doc_mainpage.h
+++ b/doxygen/input/doc_mainpage.h
@@ -22,7 +22,7 @@
  */
 
 /**
- * @mainpage mbed TLS v2.27.0 source code documentation
+ * @mainpage mbed TLS v2.28.0 source code documentation
  *
  * This documentation describes the internal structure of mbed TLS.  It was
  * automatically generated from specially formatted comment blocks in