Deprecating `x86_64-apple-darwin`

[hinto-janai]

What

Apple, GitHub, and Rust are deprecating support for x64 macOS:

• https://github.blog/changelog/2025-07-11-upcoming-changes-to-macos-hosted-runners-macos-latest-migration-and-xcode-support-policy-updates/#macos-13-is-closing-down
• https://blog.rust-lang.org/2025/09/18/Rust-1.90.0/#demoting-x86-64-apple-darwin-to-tier-2-with-host-tools

Cuprate will have to update CI and the user book:

cuprate/.github/workflows/ci.yml

Line 101 in 267c98b

macos-13, # EOL = 2025-10-24 | For x64.

cuprate/books/user/src/getting-started/download.md

Line 7 in 267c98b

| macOS x86_64 | <https://github.com/Cuprate/cuprate/releases/download/cuprated-0.0.6/cuprated-0.0.6-x86_64-apple-darwin.tar.gz>