.to_json calls should be autocorrected to .dump

What does this MR do and why?

Describe in detail what your merge request does and why.

Due to weird differences between different JSON adapters, using Gitlab::Json.dump is actually the closest match in terms of functionality to .to_json calls on objects.

Related #217633 (closed)

Branched off !101661 (closed), which is a merge dependency.

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