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/input/doc_mainpage.h b/doxygen/input/doc_mainpage.h
index 4eff836..487faf8 100644
--- a/doxygen/input/doc_mainpage.h
+++ b/doxygen/input/doc_mainpage.h
@@ -24,7 +24,7 @@
  */
 
 /**
- * @mainpage mbed TLS v2.17.0 source code documentation
+ * @mainpage mbed TLS v2.18.0 source code documentation
  *
  * This documentation describes the internal structure of mbed TLS.  It was
  * automatically generated from specially formatted comment blocks in