[CI] Mac jar build uses a different LLVM version than Linux

### Describe the bug, including details regarding any error messages, version, and platform.

The Mac build uses an older LLVM version (15) than Linux (18). This seems to be related to the CI code and the fact that the mac github runners have llvm installed via brew
https://github.com/actions/runner-images/blob/releases/macos-14-arm64/20250915/images/macos/macos-14-arm64-Readme.md
Even newer runners only have LLVM 17.

The discrepancy could lead to hard to find bugs or performance issues.

Is it possible to use vcpkg for the mac builds? That way we could be assured that they are always consistent across platforms.

A few related changes I found:
Unpin the llvm version in arrow/cpp/Brewfile. I think this might mean that theoretically the latest llvm could be installed but since the runner image already has an older one installed, that is used instead.
https://github.com/apache/arrow/commit/c1f995226c46c2bba9c5fbf988cf9b247f87adad

A related change talking about llvm 14. https://github.com/apache/arrow/pull/14310

macOS と Linux の jar-build の CI 定義を見つけ、LLVM の選択方法を比較し、その後 arrow/cpp/Brewfile とリンク先の runner-image の詳細を調べます。ビルドが一貫した LLVM バージョンを使用するか、プラットフォーム間のバージョン差異が明示的に文書化され、正当化されていれば完了です。

索引モデルが issue の本文から書いたものです。