Replace multi_json gem with standard Ruby json gem

[larouxn]

Description

Proposing we remove the multi_json gem with the standard Ruby json gem as it's now as fast or faster than other JSON gems in the majority of cases. Additionally allows us to simplify code quite a bit too. ✨

Related efforts

• Change the default serializer to the json gem for 4.0.0-beta.5 opensearch-project/opensearch-ruby#290
• Remove oj and its initializer and add json to the Gemfile mastodon/mastodon#37752

Implementation

The one quirk is apparently MultiJson.load (presumably through oj) could be passed almost anything (nil, '', IO, StringIO, etc.) and it would just return nil if it couldn't load it. JSON on the hand will error if you pass improper input. Thus, I added checks for nil, empty, and ensured if an IO or StringIO is passed it is read into a String before being passed to load.