feat: Default stackableVersion to operator version

[sbernauer]

Description

Part of stackabletech/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

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1. Changes are OpenShift compatible
2. CRD changes approved
3. Helm chart can be installed and deployed operator works
4. Integration tests passed (for non trivial changes)

Reviewer

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1. Code contains useful comments
2. (Integration-)Test cases added
3. Documentation added or updated
4. Changelog updated
5. Cargo.toml only contains references to git tags (not specific commits or branches)

Acceptance

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1. Feature Tracker has been updated
2. Proper release label has been added

[razvan]

lgtm!

[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?