ci: Change 'npm audit' failure behaviour

The audit fails only if the results include a vulnerability with a level of
at least 'high' in scheduled pipelines.

.gitlab-ci.yml

@@ -37,7 +37,7 @@ audit:
 scheduled-audit:
   stage: audit
   script:
-    - npm audit
+    - npm audit --audit-level=high
   only:
     - schedules