Update default image for S3 worker

https://redmine.teklia.com/issues/6067

The image at registry.gitlab.teklia.com/arkindex/workers/import is being moved to registry.gitlab.teklia.com/arkindex/workers/import/s3. I already updated the docs MR, doc!151 (merged).

changed milestone to %Arkindex 1.6.2

requested review from @babadie

assigned to @erouchet

merged