Build and platform requirements for Node.js 20

Build and platform requirements for Node.js 20

As discussed in the last Release WG meeting, we should soon agree on the set of platforms and build toolchains supported by v20.0.0.

Current tables are in https://github.com/nodejs/node/blob/c2e4b1fa9ad0b744616c4e4c13a5017772a630c4/BUILDING.md#supported-platforms

Main topics:

Minimum supported version of macOS
Minimum supported version of Windows
Drop support for 32-bit Windows?
Minimum version of Xcode (should support C++20)

@nodejs/tsc @nodejs/build @nodejs/releasers