[Fix] Deprecation warning on Ruby 3.4

[ndemianc]

Fixes #558

The temporarily fix was added 2 years ago at 3eb5ce2 at ruby-openai version 3.2.0.
The Upgrading guide from that time was also removed from README.md as well, so I believe it is already save to remove the compatibility fix.

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[ndemianc]

@alexrudall please review, when You have time.

[alexrudall]

Will do, I want to include it in v8 along with #566. Thanks for the PR