Switch from MinIO to Silo in integration tests

The MinIO Docker images have been unpublished, which leads to test failures on our side as we relied on it in tests. To fix them, we should move to Silo (https://silo.pgsty.com/), a maintained MinIO fork. Eventually, we should add/move to another server implementation but that's a bigger project as most S3-compatible object storage implementations don't support conditional PUT requests on which we currently depend.