Hardcode Gitlab::Json methods instead of dependency

What does this MR do and why?

Describe in detail what your merge request does and why.

Simplifies this rubocop cop to not have any external dependencies, as it's causing further problems in open MRs with additional dependencies in the Gitlab::Json class.

Follow-up from !101637 (merged)

Related #217633 (closed)

Related #378496 (closed)

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