doc: restore minimum Xcode version for macOS
https://github.com/nodejs/node/pull/36691 accidentally reverted the change to the minimum version of Xcode for macOS.
Refs: https://github.com/nodejs/node/commit/4268fae04acc16b34fb302d63b01a85725ef2043#r49622017 Refs: https://github.com/nodejs/node/pull/37872
cc @targos