nested_form uses unfrozen literal strings — will break in Ruby 4.0

Hi, thanks so much for maintaining rails_admin!

I wanted to bring up a concern with gem "nested_form", which has been archived since 2021 and isn’t maintained anymore. When using it with Ruby 3.4 and enabling:

Warning[:deprecated] = true

you’ll see warnings like:

vendor/bundle/ruby/3.4.0/gems/nested_form-0.3.2/lib/nested_form/builder_mixin.rb:92: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)

This means nested_form contains mutable string literals, which won’t be allowed in Ruby 4.0 where string literals will be frozen by default.

Since nested_form is no longer maintained, this likely won't be fixed upstream. That makes its use in rails_admin a potential blocker for Ruby 4.0 compatibility.

Would it be possible to replace nested_form with a maintained alternative, or reimplement the nested form functionality using a more Rails-native approach (e.g., fields_for combined with Stimulus or custom JavaScript)?

It would be great if this could be addressed ahead of Ruby 4.0. Thanks again for all your work on this project!