Breaking Change: Remove deprecated bazel/system_python.bzl alias.

Remove direct references to system_python.bzl in favor of using protobuf_deps.bzl instead. Use python/dist/system_python.bzl where it was moved in v5.27.0 if you need a direct reference.

See https://protobuf.dev/news/2024-10-02/#python-remove-alias

PiperOrigin-RevId: 686129383

Author: zhangskz