Preconfigure Purge to use alternate database storage on install

We cannot wait for config import to do this as installing the module
will try to use the purge queue before config import is run.

Consequently we have to set the value manually in an update hook.

10032 has already been applied on staging with release 2025.3.1 so we
need to add a new update hook for this.

web/modules/custom/dpl_update/dpl_update.install

@@ -367,3 +367,12 @@ function dpl_update_update_10031(): string {
 function dpl_update_update_10032(): string {
   return _dpl_update_install_modules(['purge_queues']);
 }
+
+/**
+ * Use alternate database backend for purge queue.
+ */
+function dpl_update_update_10033(): string {
+  // @see https://git.drupalcode.org/project/purge_queues/-/blob/2.0.x/src/Plugin/Purge/Queue/AltDatabaseQueue.php?ref_type=heads#L12
+  \Drupal::configFactory()->getEditable('purge.plugins')->set('queue', 'database_alt')->save();
+  return 'Configured purge queue to use database_alt.';
+}