Path-style S3 access is deprecated

It was recently [announced](https://forums.aws.amazon.com/ann.jspa?annID=6776) that path-style access to S3 is deprecated will be rejected by S3 starting 2020-09-30. However, since https://github.com/elastic/elasticsearch/pull/34721 (i.e. 7.0), the `repository-s3` plugin exclusively uses path-style access, to make it easier to use Minio-backed repositories. This needs a re-think.