Remove secret migration job entirely

[nightkr]

Description

Fixes #477

This was a temporary migration tool for upgrading from 24.3. Anyone
upgrading from 24.3 will need to first upgrade to 24.7 or 24.11 before
25.3 (or do the step manually). Bear in mind that we don't officially
support skipping releases when upgrading.

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. Helm chart can be installed and deployed operator works
2. Integration tests passed (for non trivial changes)

Reviewer

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1. Code contains useful comments
2. Code contains useful logging statements
3. (Integration-)Test cases added
4. Documentation added or updated. Follows the style guide.
5. Changelog updated
6. 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
3. Roadmap has been updated

[sbernauer]

Please also let @siegfriedweber have a look, as he created #536 recently for some reason?

[nightkr]

Release note:

Removed migration job for upgrading from 24.3. Upgrading directly from 24.3 is not supported.