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

[Tulio Magno Quites Machado Filho via llvm-commits]

tuliom wrote:

Acording to this [document](https://docs.github.com/en/organizations/organizing-members-into-teams/changing-team-visibility), a visible team can be viewed and mentioned by every organization member.
Secret teams get a "Secret" label that members can see.
I don't see this label in the llvm-security-group team.

The problem might be related to not having write access.

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