[ruby/date] Fix envutil for old Ruby

https://github.com/ruby/date/commit/57e25c406a

tool/lib/envutil.rb

@@ -53,7 +53,7 @@ module EnvUtil
       @original_internal_encoding = Encoding.default_internal
       @original_external_encoding = Encoding.default_external
       @original_verbose = $VERBOSE
-      @original_warning = %i[deprecated experimental].to_h {|i| [i, Warning[i]]} if RUBY_VERSION > "2.7"
+      @original_warning = defined?(Warning.[]) ? %i[deprecated experimental].to_h {|i| [i, Warning[i]]} : nil
     end
   end
 
@@ -211,7 +211,7 @@ module EnvUtil
   ensure
     stderr, $stderr = $stderr, stderr
     $VERBOSE = EnvUtil.original_verbose
-    EnvUtil.original_warning.each {|i, v| Warning[i] = v}
+    EnvUtil.original_warning&.each {|i, v| Warning[i] = v}
   end
   module_function :verbose_warning