Don't use deprecated #to_default_s in Date#to_fs

In Rails 7.1.1, using `Date#to_fs` with an un-exist format, shows the
deprecation warning like the following.

```ruby
Date.current.to_fs(:doesnt_exist)
=> DEPRECATION WARNING: to_default_s is deprecated and will be removed from Rails 7.2 (use to_s instead) (called from <main> at ./bin/rails:4)
```

But other date-time-related classes still allow to use this.

https://github.com/rails/rails/blob/6f6c211f4781e7bda63e5f47089f9d2612dc0a52/activesupport/lib/active_support/core_ext/time/conversions.rb#L57
https://github.com/rails/rails/blob/6f6c211f4781e7bda63e5f47089f9d2612dc0a52/activesupport/lib/active_support/core_ext/date_time/conversions.rb#L39

So I thought this was just a typo and fixed to use `#to_s` like
other classes.

Author: y-yagi

activesupport/lib/active_support/core_ext/date/conversions.rb

@@ -52,7 +52,7 @@ def to_fs(format = :default)
         strftime(formatter)
       end
     else
-      to_default_s
+      to_s
     end
   end
   alias_method :to_formatted_s, :to_fs

activesupport/test/core_ext/date_ext_test.rb

@@ -37,6 +37,7 @@ def test_to_fs
     assert_equal "2005-02-21",          date.to_fs(:inspect)
     assert_equal "21 Feb 2005",         date.to_fs(:rfc822)
     assert_equal "2005-02-21",          date.to_fs(:iso8601)
+    assert_equal date.to_s,             date.to_fs(:doesnt_exist)
     assert_equal "21 Feb",              date.to_formatted_s(:short)
   end