https://github.com/rails/rails/commit/3d00c8b97f3b18d89cdeaebe3d07661300ab14e4 introduced a regression:

```ruby
ActiveSupport::Inflector.inflections(:en) do |inflect|
  inflect.acronym("API")
end
```

If you have a controller called `APIController`, you may get a crash backtrace that looks like this:

```
FrozenError: can't modify frozen String: "API"
    rails (b8a399cd) actionpack/lib/action_dispatch/http/request.rb:89:in `controller_class_for'
    rails (b8a399cd) actionpack/lib/action_dispatch/request/utils.rb:100:in `action_encoding_template'
    rails (b8a399cd) actionpack/lib/action_dispatch/request/utils.rb:85:in `encode'
    rails (b8a399cd) actionpack/lib/action_dispatch/request/utils.rb:45:in `set_binary_encoding'
    rails (b8a399cd) actionpack/lib/action_dispatch/http/parameters.rb:68:in `path_parameters='
```

And you can see the issue in action like this:

```ruby
Tanda @ Rails 7.1.0.alpha (test) :004 > "API".freeze.underscore.frozen?
false # this is expected
Tanda @ Rails 7.1.0.alpha (test) :004 > "API".freeze.underscore.camelize.frozen?
false # this is *not* expected, and is what causes the above code to crash
```

I think the correct behaviour is for the inflector to always return non-frozen strings, even if it's using a cached frozen string as is the case in https://github.com/rails/rails/commit/3d00c8b97f3b18d89cdeaebe3d07661300ab14e4

Other semi-related PRs: https://github.com/rails/rails/pull/41174, https://github.com/rails/rails/pull/41881
cc @amatsuda @byroot