chore: remove github.com/google/go-github/v50 dependency

[gnuletik]

Hi !

Thanks for your project!
We are trying to use it in our project and I've seen that the transient dependency to Github was easy to remove so I made a PR.

I've also applied some general recommendations / deprecated features:

• Add cancelable context in the generator
• Replaced deprecated +build directive
• Replaced deprecated io/util package
• Removed unused generator.fatal method

Let me know if you need more changes.

Thanks!