deploy: backup job without the attachment volume's tar once the store is s3

tsk-2wj75yfph9zf open

Follow-up to tsk-enpbgadqgmbr (#1281, #1282), as agreed with the lead.

Once a deployment runs on attachments.store: s3 and no longer mounts the attachment volume (step 5 in deploy/helm/maity-hub/README.md, Moving attachments to S3), deploy/k3s-single/backup.yaml must no longer tar the volume. The bucket's versioning (and its replication) takes the place of the tar. The dump marker no longer demands the tar either (DUMP_REQUIRES_ATTACHMENTS).

Condition: until this card is merged, the volume stays mounted and in the backup, so there is no window without a backup. Removing the volume from the deployment comes only after this PR.

Scope:

backup.yaml: the attachment tar only where a volume exists, or a variant for s3. Settle the design in the PR.
The DUMP_REQUIRES_ATTACHMENTS documentation in the chart README.
restore-attachments.yaml and the restore docs: for s3, a restore from bucket versions.
KTW kubernetes.md 9111cb2f / replicas.md a70d7269.