Add feature_category to Gitlab::MergeRequests::MessageGenerator

As part of our testing best practices guidelines, we should add
`feature_category` to specs when possible.

This class is owned by the Code Review team so adding `code_review`
as `feature_category`.

spec/lib/gitlab/merge_requests/message_generator_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-RSpec.describe Gitlab::MergeRequests::MessageGenerator do
+RSpec.describe Gitlab::MergeRequests::MessageGenerator, feature_category: :code_review do
   let(:merge_commit_template) { nil }
   let(:squash_commit_template) { nil }
   let(:project) do