[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:

This should also help to solve the issue mentioned in https://github.com/llvm/llvm-project/pull/106112#pullrequestreview-2263955102

The `llvm/llvm-security-group` is larger than the Github limit for reviewers (15), making it impossible to add everybody.

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