Build extensions for Ruby 2.7 (#7027)

* Build extensions for Ruby 2.7

* Try installing bundler 2.x

* Try bumping rake-compiler-dock

* Use standard RCD images

* Avoid 'rake cross native' with rake-compiler-dock

* Use Ruby 2.5 for building Ruby <= 2.6

* Use rake-compiler 1.1.0

* Specify target

* Don't update Ruby test image for now
diff --git a/ruby/Rakefile b/ruby/Rakefile
index ad70e31..b0b803d 100644
--- a/ruby/Rakefile
+++ b/ruby/Rakefile
@@ -70,13 +70,18 @@
 
   task 'gem:windows' do
     require 'rake_compiler_dock'
-    RakeCompilerDock.sh "bundle && IN_DOCKER=true rake cross native gem RUBY_CC_VERSION=2.6.0:2.5.0:2.4.0:2.3.0"
+    ['x86-mingw32', 'x64-mingw32', 'x86_64-linux', 'x86-linux'].each do |plat|
+      RakeCompilerDock.sh <<-"EOT", platform: plat
+        bundle && \
+        IN_DOCKER=true rake native:#{plat} pkg/#{spec.full_name}-#{plat}.gem RUBY_CC_VERSION=2.7.0:2.6.0:2.5.0:2.4.0:2.3.0
+      EOT
+    end
   end
 
   if RUBY_PLATFORM =~ /darwin/
     task 'gem:native' do
       system "rake genproto"
-      system "rake cross native gem RUBY_CC_VERSION=2.6.0:2.5.1:2.4.0:2.3.0"
+      system "rake cross native gem RUBY_CC_VERSION=2.7.0:2.6.0:2.5.1:2.4.0:2.3.0"
     end
   else
     task 'gem:native' => [:genproto, 'gem:windows']