Make Brian Williams a maintainer of security-report-schemas

Justification

We need more Threat Insights maintainers for the security-report-schemas project and I already have enough experience with the project to be one.

Before Merging (Manager Tasks)

• Close any relevant trainee maintainer issues with a comment indicating that this merge request is being created, as (they are no longer required to become a maintainer).
• Mention the maintainers from the given specialty with the template below.
• Leave this merge request open for 1 week, to give the maintainers time to provide feedback.
• Ensure we have at least 2 approvals from existing maintainers.

Template call to action

SPECIALITY Maintainers, please review this proposal to make TRAINEE maintainer of PROJECT. 

* If you have blocking feedback adhering [to the documentation](https://about.gitlab.com/handbook/engineering/workflow/code-review/#how-to-become-a-project-maintainer) please share it with me via Slack.
* If you are in agreement please approve or give a 👍 on the MR

After 1 week, if there is no blocking feedback and at least 2 approvals, I will merge this MR.

Once This MR is Merged

1. Join the #backend_maintainers Slack channel
2. Ask the maintainers in your group to invite you to any maintainer-specific meeting if one exists.
3. Let a maintainer add you to @gitlab-org/maintainers/security-report-schemas with Owner access level.
4. Announce it everywhere in #sec-section
5. Familiarize yourself with documentation for Merging a merge request
6. Keep reviewing, start merging 🤘 😎 🤘