Update ruby gemspec required ruby version to >=2.7

Author: zhangskz

ruby/google-protobuf.gemspec

@@ -18,7 +18,7 @@ Gem::Specification.new do |s|
     s.files     += Dir.glob('ext/**/*')
     s.extensions= ["ext/google/protobuf_c/extconf.rb"]
     s.add_development_dependency "rake-compiler-dock", "= 1.2.1"  end
-  s.required_ruby_version = '>= 2.5'
+  s.required_ruby_version = '>= 2.7'
   s.add_development_dependency "rake-compiler", "~> 1.1.0"
   s.add_development_dependency "test-unit", '~> 3.0', '>= 3.0.9'
 end