feat: Support Ruby 4.0

[sue445]

Currently, the latest version of Ruby is the 4.0 series.

Running the code on Ruby 4.0.x results in the following error:

$ toys ci
/Users/sue445/workspace/github.com/GoogleCloudPlatform/functions-framework-ruby/lib/functions_framework.rb:15: warning: logger used to be loaded from the st
You can add logger to your Gemfile or gemspec to fix this error.
/Users/sue445/.rbenv/versions/4.0.2/lib/ruby/4.0.0/bundled_gems.rb:60:in 'Kernel.require': cannot load such file -- logger (LoadError)

$ toys ci
test/test_function.rb:16: 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.
/Users/sue445/.rbenv/versions/4.0.2/lib/ruby/4.0.0/bundled_gems.rb:60:in 'Kernel.require': cannot load such file -- ostruct (LoadError)

This is caused by logger and ostruct being promoted from default gems to bundled gems in Ruby 4.0.0.

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

Therefore, I have added these gems as runtime dependencies.

[sue445]

I encountered a conflict with the main branch, so I have rebased and force-pushed the changes.
However, the following error is occurring in my repository's CI:

Run ruby/setup-ruby@472790540115ce5bd69d399a020189a8c87d641f
Error: Error: Unknown version 4.0 for ruby on ubuntu-24.04
        Available versions for ruby on ubuntu-24.04: 1.9.3-p551, 2.0.0-p648, 2.1.9, 2.2.10, 2.3.0, 2.3.1, 2.3.2, 2.3.3, 2.3.4, 2.3.5, 2.3.6, 2.3.7, 2.3.8, 2.4.0, 2.4.1, 2.4.2, 2.4.3, 2.4.4, 2.4.5, 2.4.6, 2.4.7, 2.4.9, 2.4.10, 2.5.0, 2.5.1, 2.5.2, 2.5.3, 2.5.4, 2.5.5, 2.5.6, 2.5.7, 2.5.8, 2.5.9, 2.6.0, 2.6.1, 2.6.2, 2.6.3, 2.6.4, 2.6.5, 2.6.6, 2.6.7, 2.6.8, 2.6.9, 2.6.10, 2.7.0, 2.7.1, 2.7.2, 2.7.3, 2.7.4, 2.7.5, 2.7.6, 2.7.7, 2.7.8, 3.0.0-preview1, 3.0.0-preview2, 3.0.0-rc1, 3.0.0, 3.0.1, 3.0.2, 3.0.3, 3.0.4, 3.0.5, 3.0.6, 3.0.7, 3.1.0-preview1, 3.1.0, 3.1.1, 3.1.2, 3.1.3, 3.1.4, 3.1.5, 3.1.6, 3.1.7, 3.2.0-preview1, 3.2.0-preview2, 3.2.0-preview3, 3.2.0-rc1, 3.2.0, 3.2.1, 3.2.2, 3.2.3, 3.2.4, 3.2.5, 3.2.6, 3.2.7, 3.2.8, 3.3.0-preview1, 3.3.0-preview2, 3.3.0-preview3, 3.3.0-rc1, 3.3.0, 3.3.1, 3.3.2, 3.3.3, 3.3.4, 3.3.5, 3.3.6, 3.3.7, 3.3.8, 3.4.0-preview1, 3.4.0-preview2, 3.4.0-rc1, 3.4.0, 3.4.1, 3.4.2, 3.4.3, 3.4.4, 3.4.5, 3.5.0-preview1, head, debug, asan, 3.4-asan
        Make sure you use the latest version of the action with - uses: ruby/setup-ruby@v1

It seems that we may need to update ruby/setup-ruby to the latest version first.