Migrate "app/assets/javascripts/vue_shared/security_reports/components/security_report_download_dropdown.vue" to GlDisclosureDropdown

! Migrate to GlDisclosureDropdown and not to GlCollapsibleListbox as there aren't one or multiple options, but rather the action of downloading scanning results!

Verification steps

1. go to security-reports pipeline security tab
2. if "Scan details" is collapsed, expand by clicking "Show details"
3. click one of the "Show results" buttons
4. verify it shows the different scan results download options in a GlDisclosureDropdown
5. click on of the download options and verify it downloads the results