[gitlab] fix condition to run release candidate workflow (#1462)

[celenechang]

What does this PR do?

Cherry-pick https://github.com/DataDog/datadog-operator/pull/1462

Motivation

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Additional Notes

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Minimum Agent Versions

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• Agent: vX.Y.Z
• Cluster Agent: vX.Y.Z

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Checklist

• [ ] PR has at least one valid label: bug, enhancement, refactoring, documentation, tooling, and/or dependencies
• [ ] PR has a milestone or the qa/skip-qa label