Ruby v4.0.0 has been officially released.
This update modifies the CI configuration to include Ruby 4.0 in the testing matrix,
ensuring compatibility and stability with the latest Ruby version.

https://www.ruby-lang.org/en/news/2025/12/25/ruby-4-0-0-released/

In Ruby v4.0.0, the 'logger' library has been removed from the standard library and is now a bundled gem.
Therefore, to use it within serverkit, it is necessary to explicitly install it by adding it as a dependency in the gemspec.

> The following bundled gems are promoted from default gems.
> logger 1.7.0
> https://www.ruby-lang.org/en/news/2025/12/25/ruby-4-0-0-released/#stdlib-updates

ref:
https://github.com/serverkit/serverkit/blob/48e2bf1de089c6e3ab14e2ea0dd24da5461f9faf/lib/serverkit/logger.rb#L3

Also, since the rubocop v1.23.0 being used depends on `ostruct`, which has been removed from the default gems, `ostruct` has also been added to the Gemfile for explicit installation.

```
gems/rubocop-1.23.0/lib/rubocop/formatter/html_formatter.rb:5: warning: ostruct used to be loaded from the standard library, but is not part of the default gems since Ruby 4.0.0.
You can add ostruct to your Gemfile or gemspec to fix this error.
rake aborted!
LoadError: cannot load such file -- ostruct (LoadError)
```

> The following bundled gems are promoted from default gems.
> ostruct 0.6.3
> https://www.ruby-lang.org/en/news/2025/12/25/ruby-4-0-0-released/#stdlib-updates