doc: update support macos version for 15.x

with our minimum target already set to 10.13 for 14.x
it doesn't make sense to claim to support anything lower.

PR-URL: nodejs#35022
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>

BUILDING.md

@@ -111,7 +111,7 @@ platforms. This is true regardless of entries in the table below.
 | Windows          | x86 (native)     | >= Windows 8.1/2012 R2          | Tier 1 (running) / Experimental (compiling) <sup>[6](#fn6)</sup> | |
 | Windows          | x64, x86         | Windows Server 2012 (not R2)    | Experimental |                                   |
 | Windows          | arm64            | >= Windows 10                   | Tier 2 (compiling) / Experimental (running) |    |
-| macOS            | x64              | >= 10.11                        | Tier 1       |                                   |
+| macOS            | x64              | >= 10.13                        | Tier 1       |                                   |
 | SmartOS          | x64              | >= 18                           | Tier 2       |                                   |
 | AIX              | ppc64be >=power7 | >= 7.2 TL02                     | Tier 2       |                                   |
 | FreeBSD          | x64              | >= 11                           | Experimental | Downgraded as of Node.js 12  <sup>[7](#fn7)</sup>     |