Add Ruby 3.4 to GitHub Actions matrix

[yahonda]

Description

This pull request adds Ruby 3.4 to GitHub Actions matrix.

Type of change

Please delete options that are not relevant.

• Refactoring (improvements to code design or tooling that don't change behaviour)

Note to other contributors

None.

Checklist:

Your PR is ready for review once the following checklist is complete. You can also add some checks if you want to.

• [ ] Tests have been added for any changes to behaviour of the code
• [ ] New and existing tests are passing locally and on CI
• [x] bundle exec rubocop reports no offenses
• [ ] RDoc comments have been updated
• [ ] CHANGELOG.md has been updated