feat: Default stackableVersion to operator version

[sbernauer]

Description

Part of https://github.com/stackabletech/issues/issues/404

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
• Please make sure all these things are done and tick the boxes

# Author
- [x] Changes are OpenShift compatible
- [x] CRD changes approved
- [x] Helm chart can be installed and deployed operator works
- [ ] Integration tests passed (for non trivial changes)

# Reviewer
- [x] Code contains useful comments
- [ ] (Integration-)Test cases added
- [ ] Documentation added or updated
- [x] Changelog updated
- [x] 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

[lfrancke]

Did this change the CRD? Is this a breaking change?

[maltesander]

Should not be breaking. You can still set the stackable version, or if left out (new) will default to operator version.

[lfrancke]

But did it change the CRD?

[lfrancke]

This needs a release note entry saying that we switched the default PullPolicy

[sbernauer]

Yeah, that sounds like a good idea. Given that we would need to touch the Changelog in every operator, would it be sufficient to mention the change in the platform release notes?