chore: Update SimpleCov in gemspec

[torinoko]

Fixes

Updated the SimpleCov version because the following error occurred when running tests on Ruby 3.4:

'SimpleCov.result_exit_status': undefined method '-' for nil (NoMethodError)

        coverage_diff = last_run[:result][:covered_percent] - covered_percent

Related

• #499

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [ ] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
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• [x] I have updated my branch with the main branch
• [ ] I have added tests that prove my fix is effective or that my feature works
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• [ ] I have added inline documentation to the code I modified

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