Move populate_scheduling_type logic

What does this MR do?

Related to #217458 (closed)

As we've discussed, we need to move populate_scheduling_type! logic from Ci::ProcessPipelineService to Ci::RetryPipelineService and execute method of Ci::RetryBuildService in order to reduce the number of calls to Redis.

Just in case, we added FF ci_ensure_scheduling_type to rollback this change easily. Created a follow-up issue for this.

1. It is unlikely that there is a bug here, but we want to add a feature flag to ensure that new pipelines can be processed correctly and retried correctly if something goes wrong
2. Only legacy pipelines that are a couple of months old would be affected by toggling the feature flag
3. Simplifying the pipeline processing and removing Redis from this equation seems to be more important than unlikely possibility of breaking retries of very old pipelines (creating a new pipeline in these project will still work).

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
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• 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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