github: CODEOWNERS to protect workflows [skip ci]

> https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners
> https://docs.github.com/en/actions/security-guides/security-hardening-for-github-actions#using-codeowners-to-monitor-changes

CODEOWNERS

@@ -0,0 +1,2 @@
+# workflows changes need admin approval
+.github/workflows/ @kamailio/admin