Fix Rails/SaveBang offenses

Changelog: other
EE: true

.rubocop_todo/rails/save_bang.yml

----
-Rails/SaveBang:
-  Exclude:
-  - ee/spec/lib/analytics/merge_request_metrics_calculator_spec.rb

ee/spec/lib/analytics/merge_request_metrics_calculator_spec.rb

@@ -60,9 +60,9 @@ RSpec.describe Analytics::MergeRequestMetricsCalculator do
 
   describe '#first_reassigned_at' do
     it 'returns earliest non-author assignee creation timestamp' do
-      merge_request.merge_request_assignees.create(assignee: merge_request.author, created_at: 5.days.ago)
-      merge_request.merge_request_assignees.create(assignee: create(:user), created_at: 3.days.ago)
-      merge_request.merge_request_assignees.create(assignee: create(:user), created_at: 1.day.ago)
+      merge_request.merge_request_assignees.create!(assignee: merge_request.author, created_at: 5.days.ago)
+      merge_request.merge_request_assignees.create!(assignee: create(:user), created_at: 3.days.ago)
+      merge_request.merge_request_assignees.create!(assignee: create(:user), created_at: 1.day.ago)
 
       expect(subject.first_reassigned_at).to be_like_time(3.days.ago)
     end