Ruby 3.4 and refactor swagger documentation modules

[moskvin]

• Aligns lint configuration with Ruby 3.4 target.

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[moskvin]

I've to drop support the old grape version to make CI happy

[moskvin]

I would suggest to dropping ruby 3.1 from CI

[moskvin]

@numbata please have a look again - I've removed some weird changes

[numbata]

@moskvin Thx for contribution!
I'll review the PR later today. Hope it is okay for you.

[numbata]

Thanks for the contribution! Very good job 👍

Main things before this can merge: the CI matrix now starts at Grape 2.4.0 but the gemspec still says >= 1.7, so either the gemspec needs updating or we should keep at least one older entry. And SwaggerRouting/SwaggerDocumentationAdder sitting at the top level is a collision risk, as now that they have their own files it's a good moment to nest them under GrapeSwagger::.

The triple lookup and test issues are in the inline comments. The dead-code comment and each_key signature are minor but easy to fix while you're already touching those lines.