doc: official macOS builds now on 10.15 + Xcode 11

PR-URL: #31459
Refs: #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>

Author: rvagg

BUILDING.md

@@ -168,7 +168,7 @@ Binaries at <https://nodejs.org/download/release/> are produced on:
 | Binary package        | Platform and Toolchain                                                   |
 | --------------------- | ------------------------------------------------------------------------ |
 | aix-ppc64             | AIX 7.1 TL05 on PPC64BE with GCC 6                                       |
-| darwin-x64 (and .pkg) | macOS 10.11, Xcode Command Line Tools 10 with -mmacosx-version-min=10.10 |
+| darwin-x64 (and .pkg) | macOS 10.15, Xcode Command Line Tools 11 with -mmacosx-version-min=10.10 |
 | linux-arm64           | CentOS 7 with devtoolset-6 / GCC 6                                       |
 | linux-armv7l          | Cross-compiled on Ubuntu 16.04 x64 with [custom GCC toolchain](https://github.com/rvagg/rpi-newer-crosstools)   |
 | linux-ppc64le         | CentOS 7 with devtoolset-6 / GCC 6 <sup>[7](#fn7)</sup>                  |