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

[Maleware]

Works on my machine! LGTM!

[sbernauer]

Started https://ci.stackable.tech/view/02%20Operator%20Tests%20(custom)/job/airflow-operator-it-custom/98/