Add page size selector to vulnerability report

What does this MR do and why?

This MR adds a page size selector to the bottom of the vulnerability report. It lets the user pick between 20, 50, and 100 items per page. You must enable both the :vulnerability_report_pagination and :vulnerability_report_page_size_selector feature flags to see it.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #356322 (closed)