Updating version.json and repo version numbers to: 21.0-rc1

Protobuf.podspec

diff --git a/Protobuf.podspec b/Protobuf.podspec
index 3a5132d..37d7fc9 100644
--- a/Protobuf.podspec
+++ b/Protobuf.podspec
@@ -5,7 +5,7 @@
 # dependent projects use the :git notation to refer to the library.
 Pod::Spec.new do |s|
   s.name     = 'Protobuf'
-  s.version  = '3.20.1'
+  s.version  = '3.21.0-rc1'
   s.summary  = 'Protocol Buffers v.3 runtime library for Objective-C.'
   s.homepage = 'https://github.com/protocolbuffers/protobuf'
   s.license  = 'BSD-3-Clause'