B2 has started rejecting uploads from rclone with the error message
Backblaze does not support the 'x-amz-acl' header
This introduces a new "unset" value. If `acl` or `bucket_acl` is set
to `unset` then rclone will omit the ACL header from the request.
See: https://forum.rclone.org/t/s3-acl-issues-on-backblaze-with-multi-part-upload/16850