Ruby: Support x64-mingw-ucrt platform

[johnnyshields]

This PR adds support for Ruby x64-mingw-ucrt platform, which is the new platform Windows Ruby Installer uses as of Ruby 3.1.

As of 3.21.0.rc.1 protobuf is not including a x64-mingw-ucrt gem package. Refer to nokogiri for the expected x64-mingw-ucrt package we should see.

Refer to this GRPC ticket for details: grpc/grpc#29666

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[johnnyshields]

I've confirmed that this successfully built the Ruby gem packages in ruby/pkg dir:

It installs on Windows Ruby 3.1

C:\workspace\protobuf>gem install --local ruby\pkg\google-protobuf-3.20.1-x64-mingw-ucrt.gem
Successfully installed google-protobuf-3.20.1-x64-mingw-ucrt
Parsing documentation for google-protobuf-3.20.1-x64-mingw-ucrt
Installing ri documentation for google-protobuf-3.20.1-x64-mingw-ucrt
Done installing documentation for google-protobuf after 1 seconds
1 gem installed

[nicolasnoble]

@haberman are you still the right contact for this package?

[haberman]

We should cherry-pick this to the release branch so that it goes out in the the 3.21.0 release.

[johnnyshields]

@haberman thanks for merging! Backport done here: #9969