Replace multi_json and oj gems with standard Ruby json gem

[larouxn]

Description

Proposing we remove the multi_json and oj gems in favour of the standard Ruby json gem as it's now as fast or faster in the majority of cases compared to oj. 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

More or less just a swap and cleanup. Nothing of particular note stands out.

[cla-checker-service]

💚 CLA has been signed

[danarnold]

This would be nice for another reason: currently, elasticsearch-ruby emits multi_json deprecation warnings with multi_json >=1.21.0 since it uses the MultiJson constant instead of MultiJSON. This would avoid having to make fixes for multi_json like in elastic-transport-ruby elastic/elastic-transport-ruby#112 and elastic/elastic-transport-ruby#125.

[larouxn]

It appears jruby 9.3 does not agree with the modern json gem requirement seemingly due to its required Ruby version, which I believe is Ruby 2.6, being lower than modern json's, which is Ruby 2.7 (source). (CI logs)

Bundler found conflicting requirements for the Ruby version:
  In Gemfile:
    Ruby

    base64 was resolved to 0.3.0, which depends on
      Ruby (>= 2.4)

    bundler was resolved to 2.2.33, which depends on
      Ruby (>= 2.3.0)

    elasticsearch was resolved to 9.5.0, which depends on
      Ruby (>= 2.6)

    elasticsearch-api was resolved to 9.5.0, which depends on
      json (~> 2.19) was resolved to 2.20.0, which depends on
        Ruby (>= 2.7)

    ostruct was resolved to 0.6.3, which depends on
      Ruby (>= 2.5)

    yard was resolved to 0.9.44, which depends on
      Ruby (>= 1.8.7)

That said, jruby 9.3 has been EOL for two years (source). Maybe we can update CI? Would be glad to open a PR to remove 9.3/add newer jruby if that's something maintainers would be open to. PR opened: #3000.

[larouxn]

Let's get CI run and if it looks good merge #3000 then continue with this PR.