build: add safety net for mac_deployment_target (#28517)

As of #28480 we now dynamically determine the LSMinimumSystemVersion
value as part of the build process.  To avoid this changing and no one
realizing we now have this assert which will trip during a Chromium
upgrade if they bump the minimum supported macOS version so we can
update our documentation appropriately.

BUILD.gn

@@ -32,6 +32,10 @@ if (is_mac) {
   import("//ui/gl/features.gni")
   import("//v8/gni/v8.gni")
   import("build/rules.gni")
+
+  assert(
+      mac_deployment_target == "10.11.0",
+      "Chromium has updated the mac_deployment_target, please update this assert, update the supported versions documentation (docs/tutorial/support.md) and flag this as a breaking change")
 }
 
 if (is_linux) {