doc: official macOS builds now on 10.15 + Xcode 11

Backport-PR-URL: https://github.com/nodejs/node/pull/32527
PR-URL: https://github.com/nodejs/node/pull/31459
Refs: https://github.com/nodejs/node/issues/29216


Reviewed-By: Christian Clauss <cclauss@me.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Ash Cripps <ashley.cripps@ibm.com>
Signed-off-by: Rod Vagg <rod@vagg.org>