Update version to 2.9.0

Bump SOVERSION for parity with 2.7.2 and 2.7.3.
diff --git a/doxygen/mbedtls.doxyfile b/doxygen/mbedtls.doxyfile
index ec7a46a..510fa85 100644
--- a/doxygen/mbedtls.doxyfile
+++ b/doxygen/mbedtls.doxyfile
@@ -28,7 +28,7 @@
 # identify the project. Note that if you do not use Doxywizard you need
 # to put quotes around the project name if it contains spaces.
 
-PROJECT_NAME           = "mbed TLS v2.8.0"
+PROJECT_NAME           = "mbed TLS v2.9.0"
 
 # The PROJECT_NUMBER tag can be used to enter a project or revision number.
 # This could be handy for archiving the generated documentation or