[SergiuLupaiescu]

Fixes #3055.

multi_json 1.21 deprecated the MultiJson constant and dump/load in favor of MultiJSON with generate/parse. The default serializer was still referencing the old constant directly, causing deprecation warnings on every serialization call.

Wraps the serializer in a module that checks for MultiJSON and uses the new API when available, falling back to the legacy one for older versions.

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[larouxn]

Noting, alternatively we can completely remove multi_json and oj.

• Replace multi_json and oj gems with standard Ruby json gem #2992