Update library version to 2.18.0

Increase the SO versions of libmbedx509 and libmbedtls due to the
addition of fields in publicly visible (non-opaque) structs:
  - mbedtls_ssl_config
  - mbedtls_ssl_context
  - mbedtls_x509_crt
diff --git a/doxygen/mbedtls.doxyfile b/doxygen/mbedtls.doxyfile
index ce58d6b..b9714c5 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 v2.17.0"
+PROJECT_NAME           = "mbed TLS v2.18.0"
 
 # The PROJECT_NUMBER tag can be used to enter a project or revision number.
 # This could be handy for archiving the generated documentation or