[ticket/12963] Revert back to "migrations" folder name for extensions

The issues that can be created with the name change are just too much

PHPBB3-12963

phpBB/install/database_update.php

@@ -181,17 +181,9 @@ $phpbb_extension_manager = $phpbb_container->get('ext.manager');
 $migrations = $phpbb_extension_manager
 	->get_finder()
 	->core_path('phpbb/db/migration/data/')
-	->extension_directory('/migration')
-	->get_classes();
-
-// @deprecated 3.1.0-RC4 (To be removed: 3.2.0)
-$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.