Add issue severity label to security issues table

What does this MR do and why?

Describe in detail what your merge request does and why.

The severity label is useful in determining the importance of a security issue during the security release.

Example table

Security issues

Issue	Master merged?	Deployed?	Backports merged?	Bot Comments	Release manager comments
https://gitlab.com/gitlab-org/security/gitlab/-/issues/866 - severity3				unassigned merge requests

---

🤖 _{This table was generated by release-tools. Please open an issue in the Delivery team issue tracker if you have any suggestions or bug reports.}

Author Check-list

• Has documentation been updated?