Update Vespa version to 8.588.8. (#4143)

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

Author: github-actions[bot]

_config.yml

@@ -107,4 +107,4 @@ defaults:
       index : true
 
 variables:
-  vespa_version: "8.587.11"
+  vespa_version: "8.588.8"