Fix cycle analytics code after improving serializers

lib/gitlab/cycle_analytics/base_stage.rb

lib/gitlab/cycle_analytics/code_event_fetcher.rb

lib/gitlab/cycle_analytics/issue_event_fetcher.rb

lib/gitlab/cycle_analytics/plan_event_fetcher.rb

lib/gitlab/cycle_analytics/review_event_fetcher.rb

lib/gitlab/cycle_analytics/stage_summary.rb

lib/gitlab/cycle_analytics/staging_event_fetcher.rb