Fix Fixes scheduled pipeline execution policy branches option

What does this MR do and why?

Fixes scheduled pipeline execution policy branches option by converting symbol key to string key. Sidekiq enforces that job arguments must be native JSON types. Using symbol keys (:branch) causes a warning because symbols are not native JSON types. As a result the option was ignored and pipelines were created for the default branch instead.

References

Related work item: #547932 (closed)

How to set up and validate locally

Follow the verification steps from #547932 (comment 2589568857)

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