(dev/core#217) Search.setting.php - Fix mismatched metadata

The setting `prevNextBackend` was introduced in PR #12665.  The PR was
originally written for an earlier version (circa 5.6) and eventually
merged in a later version (circa 5.9). The metadata should match
the version-number of the actual release.

settings/Search.setting.php

@@ -208,7 +208,7 @@
       //'class' => 'crm-select2',
     ),
     'default' => 'default',
-    'add' => '5.6',
+    'add' => '5.9',
     'title' => 'PrevNext Cache',
     'is_domain' => 1,
     'is_contact' => 0,