Don't dry-run rollback-release workflow on release branches

[mbg]

To avoid the issue observed in https://github.com/github/codeql-action/actions/runs/18004376110/job/51221112307#step:7:35

Risk assessment

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• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.

[Copilot]

Pull Request Overview

This PR prevents the rollback-release workflow from running dry-runs on release branches to avoid conflicts where tags determined by the "Prepare release" job already exist.

• Adds branches-ignore configuration to exclude release branches from dry-run execution
• Addresses workflow failure observed in the linked GitHub Actions run