[BE] Remove FF and ensure rspec is passing
The GraphQL FF was disabled for existing RSpecs as the the migration is WIP. Once the migration is completed, we should re-enable the FF and make sure the tests are passing. If no update specs where needed.
Removing the stub `mr_security_widget_graphql`:
- `ee/spec/features/merge_request/user_sees_security_widget_spec.rb`
- `qa/qa/specs/features/ee/browser_ui/18_security_risk_management/dismissed_vulnerabilities_in_security_widget_spec.rb`
- `qa/qa/specs/features/ee/browser_ui/18_security_risk_management/vulnerability_management_spec.rb`
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