Description

1. Add concurrent-ruby to the main Gemfile.
2. Fixup test_web_concurrency_with_concurrent_ruby_available
3. Add test_web_concurrency_with_concurrent_ruby_unavailable
4. Remove files in test/web_concurrency_test folder

Your checklist for this pull request

• I have reviewed the guidelines for contributing to this repository.
• I have added (or updated) appropriate tests if this PR fixes a bug or adds a feature.
• My pull request is 100 lines added/removed or less so that it can be easily reviewed.
• If this PR doesn't need tests (docs change), I added [ci skip] to the title of the PR.
• If this closes any issues, I have added "Closes #issue" to the PR description or my commit messages.
• I have updated the documentation accordingly.
• All new and existing tests passed, including Rubocop.