Fix EE namespacing of DesignsCounterService

What does this MR do?

Although all tests were passing, SaveDesignsService would error in development environment when attempting to increment the counter added in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14779 with:

NameError (uninitialized constant EE::Gitlab::UsageCounters):
  ee/app/services/design_management/save_designs_service.rb:70:in `block in build_repository_action'
  ee/app/services/design_management/save_designs_service.rb:32:in `success'
  ee/app/services/design_management/save_designs_service.rb:21:in `execute'
  ee/app/graphql/mutations/design_management/upload.rb:19:in `resolve'

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers