Bump the podspec iOS min to 9.0

Xcode 12+ issue a warning when targeting older iOS versions, so avoid
the issue. #7980 updated the Xcode projects already.

Protobuf.podspec

diff --git a/Protobuf.podspec b/Protobuf.podspec
index 01a9772..d05b91b 100644
--- a/Protobuf.podspec
+++ b/Protobuf.podspec
@@ -34,7 +34,7 @@
   s.user_target_xcconfig = { 'GCC_PREPROCESSOR_DEFINITIONS' => '$(inherited) GPB_USE_PROTOBUF_FRAMEWORK_IMPORTS=1' }
   s.pod_target_xcconfig = { 'GCC_PREPROCESSOR_DEFINITIONS' => '$(inherited) GPB_USE_PROTOBUF_FRAMEWORK_IMPORTS=1' }
 
-  s.ios.deployment_target = '7.0'
+  s.ios.deployment_target = '9.0'
   s.osx.deployment_target = '10.9'
   s.tvos.deployment_target = '9.0'
   s.watchos.deployment_target = '2.0'