[mac] Update minimum macOS version to 10.12

Supporting 10.9 is becoming difficult and 10.12 is a more reasonable
baseline for modern features.

This is to avoid build error in
https://ci.chromium.org/ui/p/gn/builders/try/mac/b8689634140536035409/overview

https://endoflife.date/macos shows that 10.12 ends its support around 6
years ago.

Bug: 484863025
Change-Id: Id3503eaa5e28526aede1eeb97dbab8816deb800d
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/21021
Commit-Queue: David Turner <digit@google.com>
Reviewed-by: David Turner <digit@google.com>

build/gen.py

diff --git a/build/gen.py b/build/gen.py
index 29085c9..eb07c22 100755
--- a/build/gen.py
+++ b/build/gen.py
@@ -531,7 +531,7 @@
       ])
 
     elif platform.is_darwin():
-      min_mac_version_flag = '-mmacosx-version-min=10.9'
+      min_mac_version_flag = '-mmacosx-version-min=10.12'
       cflags.append(min_mac_version_flag)
       ldflags.append(min_mac_version_flag)
     elif platform.is_aix():