[Build] Make gRPC build with boringssl at HEAD

Previous work https://github.com/grpc/grpc/pull/41754 to build boringssl at master branch.

I've already upgrade boringssl to `0.20250415.0`, the main blocker with upgrading it to HEAD is that our test environment uses cmake 3.18 so I can't really make CI pass. For reference, latest boringssl requires cmake 3.22.

Ideally we want to upgrade to debian 12 or above which natually comes with cmake 3.25. (Debian 11 is out of LTS soon anyway at 2026/08/31). Alternatively we can use debian archive to use backports on oldoldstable.