macOS future strategy

Currently the project builds three macOS packages available to download from nodejs.org:

• macOS arm64 tarball
• macOS x64 tarball
• macOS universal binary pkg (i.e. the same installer installs either arm64 or x64).

To build the pkg we build Node.js twice on an arm64 macOS VM:

• Once for x64 under Rosetta 2
• Once for arm64 (native)

and then combine the two into the pkg, sign/notarize.

Apple have announced that the last release of macOS that will support Rosetta 2 will be macOS 27. That's the version two on from what we're currently on (macOS 15 -- Apple renumbered so the sequence goes macOS 15, macOS 26, macOS 27). The same Apple announcement said that the last macOS version to support x64 is Tahoe (26).

So we have two decisions:

1. At some point we make the pkg builds arm64 only (as we currently need Rosetta 2 to build those without significant rework of how the releases are done).
2. At some point we will need to drop the x64 macOS version of Node.js completely.

Planning is a little difficult with macOS because Apple do not usually announce End-of-Life dates... they just stop releasing after a while. Also they have made changes before around signing/gatekeeper which forced Xcode updates that in turn forced macOS version updates. So there's a large element of "our best guess" when trying to work out what we're likely to be able to support for the duration of a Node.js release line.

Starting the discussion now as I think our preference would be to make changes on semver major boundaries (unless Apple forces our hand as with the previously mentioned signing/gatekeeper change). So discuss first wrt. Node.js 27 (2027) and if we decide that is too soon, switch focus to Node.js 28 etc.

Refs: https://github.com/nodejs/node/pull/63055

Start with this issue and linked PR #63055, reviewing the current macOS package and release approach described here. Evaluate the proposed Node.js 27 timing against Node.js 28 and the stated Apple constraints. Done means the project has an agreed strategy for the universal pkg and x64 support.