Extract Ci::CancelPipelineService class

What does this MR do and why?

The Pipeline#cancel_running method had a complex logic and private methods it depended on. This logic required to be extracted into a service object.

With this extraction we also improve the response object for better testing.

Extract Ci::CancelPipelineService class

Move all logic related to canceling a pipeline to a dedicated service object.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.