Bump version to 2.28.5

```
./scripts/bump_version.sh --version 2.28.5
```

Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>
diff --git a/doxygen/input/doc_mainpage.h b/doxygen/input/doc_mainpage.h
index d19f93e..0cad01b 100644
--- a/doxygen/input/doc_mainpage.h
+++ b/doxygen/input/doc_mainpage.h
@@ -22,7 +22,7 @@
  */
 
 /**
- * @mainpage Mbed TLS v2.28.4 API Documentation
+ * @mainpage Mbed TLS v2.28.5 API Documentation
  *
  * This documentation describes the internal structure of Mbed TLS.  It was
  * automatically generated from specially formatted comment blocks in