Document in 5.9.0 release notes that macOS support is being dropped to Tier 3 in 5.10.0

[str4d]

Intel macOS has been officially at Tier 3 for a long time. We've been trying to raise it to Tier 1 by ensuring we run builds and tests for it; the last thing that was missing was packages. However, Rust 1.70 (released June 2023) migrated to a version of LLVM for which no Intel macOS packages have been built by the LLVM maintainers / ecosystem, which means we are unable to both support building on Intel macOS, and upgrade Rust.

Given that zcashd is now on the path to deprecation, and the Intel Macbooks were discontinued in 2021, we are no longer going to try and get Intel macOS above Tier 3. The code for it will remain in the codebase, but we will no longer (natively) build it in CI (we might continue to natively test it if we can easily get cross-compilation working in CI, but that will not be a blocker), and native builds will immediately start failing once we upgrade Clang and Rust.

We should notify users of this in the 5.9.0 release notes.