Set projects_post_creation_worker feature flag to default

What does this MR do?

• Contributes to #326665 (closed)
• Introduced in !58119 (merged)

Use Projects::PostCreationWorker to asynchronously setup PrometheusService by default.

Feature flag is enabled globally.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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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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