Renamed migrations_templates directory to migrations

The core migration templates have moved from MODULENAME/migration_templates to MODULENAME/migrations directory, in accordance with #2625696: Make migrations themselves plugins instead of config entities. In that issue, the canonical place for migration YAMLs became the MODULENAME/migrations directory but the core migrations remained MODULENAME/migration_templates.

Core migrations are now also updated to use MODULENAME/migrations. For backwards compatibility, the MODULENAME/migration_templates directory still can be used, so no change is required for contrib or custom modules, but this fallback will be removed before 9.0.0 so you should update your code when appropriate.