doc/rl-2511: announce projected `x86_64-darwin` deprecation

macOS 27 is going to drop Intel support, so we’re pre‐announcing
the inevitable so that people can prepare.

We already announced that we’re aligning our OS support policy with
Apple’s starting in 25.11, so it’s very unlikely that we could
justify making an exception to devote resources to `x86_64-darwin`
support in 28.11 and beyond, after Apple stop releasing security
updates for the platform.

However, the end of support in Nixpkgs may come sooner than that. Apple
have announced that [Rosetta 2 will be pared down] by macOS 28 to not
support emulation of arbitrary applications. We use `aarch64-darwin`
builder machines exclusively and rely on Rosetta 2 to build packages
for `x86_64-darwin`. As we try to keep the builders on the latest OS
versions, that would mean that we’d lose the ability to build for
`x86_64-darwin` around the release of 27.11, unless we held back on
updating the OS on the builders for a year.

[Rosetta 2 will be pared down]: https://developer.apple.com/documentation/apple-silicon/about-the-rosetta-translation-environment

Additionally, `x86_64-darwin` is the slowest system to build due to
our limited Mac builder resources and the emulation overhead. Dropping
support will more than double our effective `aarch64-darwin` build
capacity and benefit the whole project by reducing the bottleneck on
world rebuilds during `staging-next` cycles. It’s hard to find good
data on the relative market share, but the May 2025 [Steam Hardware
Survey] shows over 80% of their macOS users already being on Apple
Silicon. Therefore, I’d personally expect us to drop support by
26.11, given the trade‐off between the resources it will take to
continue supporting `x86_64-darwin` and the number of users it is
likely to benefit. (And I’m typing this on a Intel Mac myself…)

[Steam Hardware Survey]: https://store.steampowered.com/hwsurvey/processormfg/

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