Set deduplicate strategy to until_executed

What does this MR do?

IssuePlacementWorker is being invoked on every issue creation. If a project creates multiple issues in a very short time(e.g. a bot or some sort of an import), the IssuePlacementWorker would be invoked multiple times for the same project. Avoid that by deduplicating the job through until_executed strategy.

re #276483 (closed) re https://gitlab.slack.com/archives/C020JDG6G0Y re gitlab-com/gl-infra/production#4321 (closed)

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