Replace deprecated ruby File.exists with File.exist and tweak tutorial

[caguero]

🦟 Bug fix

While testing the logging tutorial , I ran into an issue executing:

gz log record --force --file tutorial.tlog

The ruby File.exists() function was removed in 3.2.0, which is the version in Ubuntu Noble. I updated the code and also used snippet in the tutorial.

Summary

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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