Issue #3379215: Use Choice config validation constraint

config/schema/csp.schema.yml

@@ -3,6 +3,9 @@
 csp.settings:
   type: config_object
   label: 'Content Security Policy Settings'
+  constraints:
+    Choice:
+      callback: \Drupal\csp\csp.module::getCoreValidVersions
   mapping:
     report-only:
       type: csp_policy