Prevent overwriting Hash#except method present in Ruby 3+

Ruby 3+ introduced `Hash#except` method natively [1], so we can skip
I18n's custom implementation here if the method is already defined.
This follows the same Rails convention [2].

Note: This seems to be causing a `stack level too deep (SystemStackError)`
with a specific set of Gemfiles under Ruby 3, reported to `SimpleForm`
originally [3].  I haven't been able to fully narrow it down after some
initial investigation, nor reproduce it without the given set of gems in
the Gemfile reported, but it seems there's some interactions between
those gems, and combined with the fact that Ancestry loads I18n early on
to setup the load path [4], is possibly causing calls to `Hash#except`
to hang or raise with the `stack level too deep` error, and after
skipping the method re-declaration here it's not reproducible anymore.

[1] https://bugs.ruby-lang.org/issues/15822
[2] https://github.com/rails/rails/blob/5f3ff60084ab5d5921ca3499814e4697f8350ee7/activesupport/lib/active_support/core_ext/hash/except.rb#L12-L14
[3] heartcombo/simple_form#1724
    steps to reproduce issue: heartcombo/simple_form#1724 (comment)
[4] https://github.com/stefankroes/ancestry/blob/71fe7042791f5943b5370240e4c6068dce73233d/lib/ancestry.rb#L9-L10

lib/i18n/core_ext/hash.rb

@@ -4,7 +4,7 @@ module HashRefinements
       using I18n::HashRefinements
       def except(*keys)
         dup.except!(*keys)
-      end
+      end unless method_defined?(:except)
 
       def except!(*keys)
         keys.each { |key| delete(key) }