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.

PR-URL: https://github.com/nodejs/node/pull/38266
Refs: https://github.com/nodejs/node/commit/4268fae04acc16b34fb302d63b01a85725ef2043#r49622017
Refs: https://github.com/nodejs/node/pull/37872


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Beth Griggs <bgriggs@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>