[llvm] [github][Security] Add the llvm-security-group team as code owner for Security.rst (PR #106255)

[Josh Stone via llvm-commits]

cuviper wrote:

I think that's still not visible outside the org, but I have no idea how to open that up further.

Or it may just be write access. I dropped my own write access because I felt that sending PRs was enough for me, but I guess I could get it back if that's important here.

https://github.com/llvm/llvm-project/pull/106255