Dropped Ruby 2.3 and 2.4 support for CI and releases.

kokoro/linux/dockerfile/test/ruby/Dockerfile

@@ -31,8 +31,6 @@ RUN gpg --keyserver hkp://keyserver.ubuntu.com --recv-keys \
     7D2BAF1CF37B13E2069D6956105BD0E739499BDB
 RUN \curl -sSL https://get.rvm.io | bash -s master
 
-RUN /bin/bash -l -c "rvm install 2.3.8"
-RUN /bin/bash -l -c "rvm install 2.4.5"
 RUN /bin/bash -l -c "rvm install 2.5.1"
 RUN /bin/bash -l -c "rvm install 2.6.0"
 RUN /bin/bash -l -c "rvm install 2.7.0"

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

@@ -67,7 +67,7 @@ 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-darwin MAKE="$MAKE"
   rake -f "$CROSS_RUBY" cross-ruby VERSION="$v" HOST=aarch64-darwin MAKE="$MAKE"

ruby/Gemfile

@@ -1,5 +1,3 @@
 source 'https://rubygems.org'
 
 gemspec
-
-gem "irb", "~> 1.1", "< 1.2.0" if RUBY_VERSION < "2.5"

ruby/Rakefile

@@ -117,15 +117,15 @@ 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=3.0.0:2.7.0:2.6.0:2.5.0:2.4.0:2.3.0
+        IN_DOCKER=true rake native:#{plat} pkg/#{spec.full_name}-#{plat}.gem RUBY_CC_VERSION=3.0.0:2.7.0:2.6.0:2.5.0
       EOT
     end
   end
 
   if RUBY_PLATFORM =~ /darwin/
     task 'gem:native' do
       system "rake genproto"
-      system "rake cross native gem RUBY_CC_VERSION=3.0.0:2.7.0:2.6.0:2.5.1:2.4.0:2.3.0"
+      system "rake cross native gem RUBY_CC_VERSION=3.0.0:2.7.0:2.6.0:2.5.1"
     end
   else
     task 'gem:native' => [:genproto, 'gem:windows', 'gem:java']