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

[Ahmed Bougacha via llvm-commits]

https://github.com/ahmedbougacha approved this pull request.

The diff warns with:
> Unknown owner on line 155: make sure the team @llvm/llvm-security-group exists, is publicly visible, and has write access to the repository

It exists and seems visible, so I assume that's another symptom of what you were discussing in #106112, namely that some of the members don't have write access?
I don't know how github will behave.  If it adds everyone else, I suppose that's better than today's situation, so I'm happy to give this a try;  LGTM!

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