Previously, S3 backups and uploads larger than the multipart threshold
crashed with `undefined method 'downcase' for nil` whenever
`AWS_REQUEST_CHECKSUM_CALCULATION` was set to `when_required` (a common
setup for S3-compatible providers such as Cloudflare R2, Backblaze B2,
and MinIO), because of a bug in `aws-sdk-s3` 1.182.0's multipart
uploader.
This change bumps `aws-sdk-s3` to 1.227.0 — which respects
`when_required` in multipart uploads — and adapts to its API changes:
moving off the newly-deprecated
`Aws::S3::Object#upload_file`/`#download_file` onto
`Aws::S3::TransferManager`, and reading the ETag back from the
destination now that multipart copies no longer return a response.