Harmonize DAST profiles mock for frontend testing

The following discussion from !48408 (merged) should be addressed:

• @pgascouvaillancourt started a discussion:

  follow-up: we now have mock data for site/scanner profiles in a few different places, we should merge all those mocks in a single source of truth. Ideally, we would use fixtures to make sure we're as close as possible to the production API. https://docs.gitlab.com/ee/development/testing_guide/frontend_testing.html#frontend-test-fixtures

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• Leverage fixtures to generate reliable mocks for DAST scans frontend specs.
  • Reminder: getJSONFixture has been removed. We now need to import the JSON directly.
• Remove existing mocks.
  • ee/spec/frontend/on_demand_scans/mocks/mock_data.js
  • ee/spec/frontend/security_configuration/dast_profiles/mocks/mock_data.js