Dev Task: S3-backup imagestream clean up

**Describe the task**
We need to clean up our use of the s3-backup build config and imagestreams.

**Acceptance Criteria**
- [ ] s3-backup build config references main branch
- [ ] s3-backup imagestream tags are pruned to 3 most recent
- [ ] s3-backup:prod imagestream tag points to most recent known good imagestream
- [ ] prod backups point to s3-backup:prod imagestream
- [ ] updates readme on s3-backup to clarify workflow for updating the s3-backup build config and proper tagging of s3-backup:dev and s3-backup:prod imagestreams

Start by locating the s3-backup build config, imagestream definitions, production backup configuration, and the s3-backup README. Trace how the build branch, image tags, and production backup image are currently connected. Done means all five acceptance criteria are met and the README explains the update and tagging workflow.

Written by the indexing model from the issue text.