GH-41397: [C#] Downgrade macOS test runner to avoid infrastructure bug (

#41934)

### What changes are included in this PR?

Downgrades the macOS test image for C# to use an older operating system. This works around pythonnet/pythonnet#2396.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #41397

Authored-by: Curt Hagenlocher <curt@hagenlocher.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

.github/workflows/csharp.yml

@@ -94,8 +94,8 @@ jobs:
  run: ci/scripts/csharp_test.sh $(pwd)
 
  macos:
- name: ARM64 macOS 14 C# ${{ matrix.dotnet }}
- runs-on: macos-latest
+ name: AMD64 macOS 13 C# ${{ matrix.dotnet }}
+ runs-on: macos-13 # Pending https://github.com/pythonnet/pythonnet/issues/2396
  if: ${{ !contains(github.event.pull_request.title, 'WIP') }}
  timeout-minutes: 15
  strategy: