Protocol Buffers v21.0-rc2

Python

• Fix windows builds
• Throw more helpful error if generated code is out of date
• Fixed two reference leaks

Ruby

• Support x64-mingw-ucrt platform

PHP

• Fix SEGV by not writing to shared memory for zend_class_entry

C#

• Suppress warning CS8981

Other

• Fix Maven release to release actual osx_aarch64 binary
• Fix protoc zips to have the proto files for well known types