refactor: Use ProductImage::product_version internally

[sbernauer]

Description

Follow-up of https://github.com/stackabletech/operator-rs/pull/817

This way we have a single point of responsibility, which makes it easier to plug in the recommended product version later on, while maintaining the ergonomic API from https://github.com/stackabletech/operator-rs/pull/817.

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
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# Author
- [ ] Changes are OpenShift compatible
- [ ] CRD changes approved
- [ ] Integration tests passed (for non trivial changes)

# Reviewer
- [ ] Code contains useful comments
- [ ] (Integration-)Test cases added
- [ ] Documentation added or updated
- [ ] Changelog updated
- [ ] Cargo.toml only contains references to git tags (not specific commits or branches)

# Acceptance
- [ ] Feature Tracker has been updated
- [ ] Proper release label has been added