Stop building binary gems for ruby <2.5. (#7453)

It is no longer possible to "bundle install" these versions:

$ bundle install
Fetching gem metadata from https://rubygems.org/.................
reline-0.1.3 requires ruby version >= 2.5, which is incompatible with the current version, ruby 2.4.9p362

kokoro/release/ruby/macos/ruby/ruby_build_environment.sh

@@ -63,7 +63,7 @@ set +x
 rvm use 2.5.0
 set -x
 ruby --version | grep 'ruby 2.5.0'
-for v in 2.6.0 2.5.1 2.4.0 2.3.0 ; do
+for v in 2.6.0 2.5.1 ; do
   ccache -c
   rake -f "$CROSS_RUBY" cross-ruby VERSION="$v" HOST=x86_64-darwin11 MAKE="$MAKE"
 done

ruby/Rakefile

@@ -73,7 +73,7 @@ else
     ['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
+        IN_DOCKER=true rake native:#{plat} pkg/#{spec.full_name}-#{plat}.gem RUBY_CC_VERSION=2.7.0:2.6.0:2.5.0
       EOT
     end
   end
@@ -81,7 +81,7 @@ else
   if RUBY_PLATFORM =~ /darwin/
     task 'gem:native' do
       system "rake genproto"
-      system "rake cross native gem RUBY_CC_VERSION=2.7.0:2.6.0:2.5.1:2.4.0:2.3.0"
+      system "rake cross native gem RUBY_CC_VERSION=2.7.0:2.6.0:2.5.1"
     end
   else
     task 'gem:native' => [:genproto, 'gem:windows']