ci: cross-compile Intel macOS from macos-14 ARM runner

GitHub retired the free macos-13 Intel runners. Build x86_64-apple-darwin
on the macos-14 host instead; Xcode on the ARM runner ships both SDKs
and the Rust toolchain action already installs the target.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Hornfisk

.github/workflows/release.yml

@@ -23,7 +23,7 @@ jobs:
           - os: macos-14
             target: aarch64-apple-darwin
             artifact: slammer-macos-arm64
-          - os: macos-13
+          - os: macos-14
             target: x86_64-apple-darwin
             artifact: slammer-macos-x86_64
           - os: windows-latest