Enable vulnerability_report_pagination feature flag by default

This MR enables the :vulnerability_report_pagination feature flag by default. It's already enabled on gitlab.com and we received positive feedback on it, so this MR will roll it out to self-hosted instances as well.

This MR also removes the :vulnerability_report_page_size_selector feature flag. Originally, the GraphQL endpoint to fetch vulnerabilities had some performance issues, so we didn't want to allow users to fetch more than 20 items per page unless it was impeding their workflow. However, we've since addressed the performance issues and it should be safe now to enable the page size selector to allow users to select up to 100 items per page.

Related to #351975 (closed) and #356888 (closed)