Correctly process Bamboo API results

What does this MR do?

Trivial fix for GitLab to process correctly Bamboo API results.

Bamboo API returns an array of results as per https://docs.atlassian.com/atlassian-bamboo/REST/6.2.5/#d2e786 (this is also true for older 5.X versions), but GitLab was not selecting an element in the array of results.

Without this fix, Bamboo integration causes the Sidekiq job morgue to fill up with failed jobs such as:

Queue 
reactive_caching
Job 
ReactiveCachingWorker
Arguments 
"BambooService", 416xxx, "9073528e58ecd742524336c2004cf13dxxxxx", "48-investigate-pm-xxxxxx"
JID 
614c9aaf2288d313705ed27d
Created At 
2 hours ago
Enqueued 
2 hours ago
Last Retry 
2 hours ago
Error
Error Class 
TypeError
Error Message 
no implicit conversion of String into Integer

What are the relevant issue numbers?

Fixes #51950 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides