Fix Gitlab::GrapeOpenapi.generate arguments

What does this MR do and why?

Fixes Gitlab::GrapeOpenapi.generate arguments:

Gitlab::GrapeOpenapi.generate passes 2 arguments to Gitlab::GrapeOpenapi::Generator#initialize, which only accepts 1. This results in an ArgumentError being raised.

This commit fixes Gitlab::GrapeOpenapi.generate by making it accept only 1 argument.

References

Resolves: #583206

Screenshots or screen recordings

Before	After

How to set up and validate locally

In the rails console:

Rails.application.eager_load!
api_classes = API::Base.descendants
entity_classes = Grape::Entity.descendants
spec = Gitlab::GrapeOpenapi.generate(api_classes: api_classes, entity_classes: entity_classes)
File.write(Rails.root.join('tmp', 'myfile.json'), JSON.pretty_generate(spec))

Confirm that:

1. No error is raised
2. <project-root>/tmp/myfile.json exists and has the expected structure

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Related to #583206