Move issues to the end on creation in the new-issue-worker

What does this MR do?

Related to #238416 (closed)

We have seen query timeouts during movement and rebalancing operations as part of Issue#move_to_end. Due to the nature of movements, we cannot offer constant time guarantees, so the best thing is to move this operation into the NewIssueWorker.

This calls Issue.move_nulls_to_end on recently created unpositioned issues, which will preserve creation order with more fidelity in the presence of out-of-order execution of multiple workers.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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