Update the macOS Podfile template platform version (#38651)

Change the target macOS version from 10.13 to 10.11, per decision on
target platform.

Part of #33200

packages/flutter_tools/templates/cocoapods/Podfile-macos

 # Using a CDN with CocoaPods 1.7.2 or later can save a lot of time on pod installation, but it's experimental rather than the default.
 # source 'https://cdn.cocoapods.org/'
 
-platform :osx, '10.13'
+platform :osx, '10.11'
 
 # CocoaPods analytics sends network stats synchronously affecting flutter build latency.
 ENV['COCOAPODS_DISABLE_STATS'] = 'true'