Update Vespa version to 8.401.18. (#3351)

Co-authored-by: kkraune <[email protected]>

Author: github-actions[bot]

_config.yml

@@ -88,4 +88,4 @@ defaults:
       index : true
 
 variables:
-  vespa_version: "8.400.15"
+  vespa_version: "8.401.18"