CI: Add Ruby 3.4, drop Ruby 3.0 (EOL)

[olleolleolle]

This PR only updates the CI matrix to the latest generally available Ruby versions.

[olleolleolle]

Hm, perhaps we have a linting issue.

Errors on modified lines:
/home/runner/work/slim-lint/slim-lint/lib/slim_lint/atom.rb:91:7: C: [Correctable] Style/RedundantCondition: Use double pipes `||` instead.

✗ One or more pre-commit hooks failed

[sds]

Thanks!