Remove useless attachToDocument usage in ee/spec/frontend/vue_shared

What does this MR do?

As described in &2419 (closed), cleans up our Jest tests from unnecessary attachToDocument usage

Involved specs

• ee/spec/frontend/vue_shared/security_reports/card_security_reports_app_spec.js
• ee/spec/frontend/vue_shared/security_reports/components/dismiss_button_spec.js
• ee/spec/frontend/vue_shared/security_reports/components/modal_footer_spec.js
• ee/spec/frontend/vue_shared/security_reports/components/dismissal_note_spec.js
• ee/spec/frontend/vue_shared/security_reports/components/event_item_spec.js
• ee/spec/frontend/vue_shared/security_reports/components/modal_spec.js

Conformity

• Merge request performance guidelines
• Style guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Closes: #196753 (closed)