Update feature category for MergeWorker
What does this MR do and why?
Problem
MergeWorker currently belongs to source_code_management
feature
category. But the worker just makes a call to
MergeRequests::MergeService
that belongs to code_review_workflow
scope. That introduces an unnecessary shared ownership of this feature.
Solution
Reassign MergeWorker to code_review_workflow
feature category.