Fix CI and add tests for latest ruby/rspec

[Drowze]

• CI: Fix usage of deprecated actions:
  • Updated actions/upload-artifact to the latest version (as v2 was throwing errors)
  • Added actions/upload-artifact/merge so the final uploaded artifact is similar to how it was on upload-artifact@v2
• Add tests for latest ruby/rspec
  • Added tests for ruby 3.2, 3.3 and 3.4
  • Added tests for RSpec 3.11, 3.12, 3.13

Notes:

• Ruby 3.2+ is not compatible with RSpec <= 3.3, so the CI workflow was updated accordingly
• I was getting too many dependency errors trying to use the same Gemfile.lock file on all ruby versions, so following Apprails's recommendation, I added gemfiles/*.lock to .gitignore.

Successful CI run: https://github.com/Drowze/rspec_junit_formatter/actions/runs/13376950471