Update Vespa version to 8.678.25. (#4672)

Co-authored-by: kkraune <bot+actions@vespa.ai>

Author: github-actions[bot]

_config.yml

@@ -108,4 +108,4 @@ defaults:
                         # This does NOT affect external search engines - use 'meta_robots_noindex: true' to add a robots noindex meta tag.
 
 variables:
-  vespa_version: "8.677.31"
+  vespa_version: "8.678.25"