Update Vespa version to 8.396.18. (#3322)

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

Author: github-actions[bot]

_config.yml

@@ -83,4 +83,4 @@ defaults:
       index : true
 
 variables:
-  vespa_version: "8.395.28"
+  vespa_version: "8.396.18"