[ruby/time] Do not redefine Time#xmlschema if it already exists

[Feature #20707]

Ruby 3.4 will define this method natively, so the time gem shouldn't
redefine it with a slower version.

https://github.com/ruby/time/commit/f05099ce38

lib/time.rb

@@ -695,6 +695,7 @@ class Time
     getutc.strftime('%a, %d %b %Y %T GMT')
   end
 
+  unless method_defined?(:xmlschema)
     #
     # Returns a string which represents the time as a dateTime defined by XML
     # Schema:
@@ -724,6 +725,6 @@ class Time
       end
       s << (utc? ? 'Z' : strftime("%:z"))
     end
-  alias iso8601 xmlschema
+  end
+  alias iso8601 xmlschema unless method_defined?(:iso8601)
 end
-