Add pipeline Security tab to Vue 3 migration

What does this MR do and why?

Opt the pipeline Security tab into Vue 3 via a new vue3_migration.yml, reusing the existing vue3_migrate_pipelines flag. It shares the pipelines/show page but had its own webpack entrypoint that was missing this config, so it stayed on Vue 2.

Also fix a crash the migration surfaced: scrollToTopOfList in vulnerability_report.vue read a template ref before it was attached under Vue 3.

References

Related #609371

How to set up and validate locally

1. Enable the flag: vue3_migrate_pipelines
2. Restart vite: gdk restart vite
3. Open a pipeline that has security scan results and switch to the Security tab. (e.g. import https://gitlab.com/gitlab-examples/security/security-reports and run a pipeline on master)
4. Confirm the security dashboard renders (findings table, filters, severity badges, finding drawer/modal).