[ruby/date] Fixed wrong minimum version of Ruby

Fixed https://github.com/ruby/date/issues/83

https://github.com/ruby/date/commit/9731a3e732

ext/date/date.gemspec

@@ -25,7 +25,7 @@ Gem::Specification.new do |s|
     s.extensions = "ext/date/extconf.rb"
   end
 
-  s.required_ruby_version = ">= 2.4.0"
+  s.required_ruby_version = ">= 2.6.0"
 
   s.authors = ["Tadayoshi Funaba"]
   s.email = [nil]