Add security policy

[carlewis]

Overview

Add security policy file.

Reason for change

Security policy informs on the vulnerability reporting channels, supported versions and disclosure policy.

Checklist

• Read and follow the project Code of Conduct.
• Make sure the project builds successfully with your changes.
• Run relevant testing locally to avoid regressions.
• Run clang-format-19 on all modified code.

[coldav]

I'm okay with this wording, but I think everyone in the team should approve.