Update version to 2.9.0

Bump SOVERSION for parity with 2.7.2 and 2.7.3.
diff --git a/doxygen/input/doc_mainpage.h b/doxygen/input/doc_mainpage.h
index 7952cbc..e27c221 100644
--- a/doxygen/input/doc_mainpage.h
+++ b/doxygen/input/doc_mainpage.h
@@ -24,7 +24,7 @@
  */
 
 /**
- * @mainpage mbed TLS v2.8.0 source code documentation
+ * @mainpage mbed TLS v2.9.0 source code documentation
  *
  * This documentation describes the internal structure of mbed TLS.  It was
  * automatically generated from specially formatted comment blocks in