Temporary set mac_deployment_target to 10.13

This needs to be rolled back as soon as the deprecated declarations
diagnostic errors get fixed.

Bug: b/288827308
Change-Id: I9584e0f156b0bd0ba40809d43e1edd7ba9ff5674
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/310300
Auto-Submit: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#40342}
diff --git a/.gn b/.gn
index ed92a1b..65b7a6b 100644
--- a/.gn
+++ b/.gn
@@ -50,6 +50,7 @@
   mac_sdk_min = "10.12"
 
   ios_deployment_target = "12.0"
+  mac_deployment_target = "10.13"
 
   # The SDK API level, in contrast, is set by build/android/AndroidManifest.xml.
   android32_ndk_api_level = 21