[ticket/12963] Fix the list of migrations in database_update.php

Without this patch the finder grab all the class available in the ext
folder and not only the migrations. This change is backported for the one
done before to the cli tool db:migrate. (see the commit
3420f8f320)

PHPBB3-12963

phpBB/install/database_update.php

@@ -181,7 +181,17 @@ $finder = $phpbb_extension_manager->get_finder();
 
 $migrations = $finder
 	->core_path('phpbb/db/migration/data/')
+	->extension_directory('/migrations')
 	->get_classes();
+
+// @deprecated to be removed in 3.2 final
+$migrations_deprecated = $phpbb_extension_manager
+	->get_finder()
+	->extension_directory('/migrations')
+	->get_classes();
+
+$migrations = array_merge($migrations, $migrations_deprecated);
+
 $migrator->set_migrations($migrations);
 
 // What is a safe limit of execution time? Half the max execution time should be safe.