set min supported iOS version to iOS 13 because of new async / await syntax

Author: haashem

ios/spotify_sdk.podspec

@@ -15,8 +15,8 @@ Unofficial Spotify Flutter SDK.
   s.source           = { :path => '.' }
   s.source_files = 'Classes/**/*'
   s.dependency 'Flutter'
-  s.platform = :ios, '9.0'
-  s.ios.deployment_target = '9.0'
+  s.platform = :ios, '13.0'
+  s.ios.deployment_target = '13.0'
   # Flutter.framework does not contain a i386 slice. Only x86_64 simulators are supported.
   s.pod_target_xcconfig = { 'DEFINES_MODULE' => 'YES', 'VALID_ARCHS[sdk=iphonesimulator*]' => 'x86_64' }
   s.swift_version = '5.0'