Bump version to 4.0.0

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/doxygen/mbedtls.doxyfile b/doxygen/mbedtls.doxyfile
index 1e494a0..917b88d 100644
--- a/doxygen/mbedtls.doxyfile
+++ b/doxygen/mbedtls.doxyfile
@@ -1,4 +1,4 @@
-PROJECT_NAME           = "Mbed TLS v3.6.0"
+PROJECT_NAME           = "Mbed TLS v4.0.0"
 OUTPUT_DIRECTORY       = ../apidoc/
 FULL_PATH_NAMES        = NO
 OPTIMIZE_OUTPUT_FOR_C  = YES