fix: Correctly migrate SECRET_KEY from 26.3 setups

[sbernauer]

Description

Fixes #753

We basically lookup connection.secretKey from the old credentials Secret and use that as the SECRET_KEY. If - for some reason, e.g. fresh setup - it doesn't exist we still create a random value (as we currently do)

We had to revert the credentialsSecret -> credentialsSecretName rename, as otherwise the operator would reconcile the CRD on startup. This would in turn result in the credentialsSecret being dropped, so we loos the information how the old Secret was called.

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

• Changes are OpenShift compatible
• CRD changes approved
• CRD documentation for all fields, following the style guide.
• Helm chart can be installed and deployed operator works
• Integration tests passed (for non trivial changes)
• Changes need to be "offline" compatible
• Links to generated (nightly) docs added
• Release note snippet added -> In feat: Add generic database connection mechanism #722 (comment)

Reviewer

• Code contains useful comments
• Code contains useful logging statements
• (Integration-)Test cases added
• Documentation added or updated. Follows the style guide.
• 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
• Links to generated (nightly) docs added
• Release note snippet added
• Add type/deprecation label & add to the deprecation schedule
• Add type/experimental label & add to the experimental features tracker

[NickLarsenNZ]

LGTM, a few nit picks.

[sbernauer]

Started https://testing.stackable.tech/view/02%20Operator%20Tests%20(custom)/job/superset-operator-it-custom/51/

[sbernauer]

Tests passed, did some manual upgrade testing

[Techassi]

Mostly lgtm, just two minor comments.