Use oj gem for faster JSON processing

What does this MR do?

Use oj gem for faster JSON processing

Are there points in the code the reviewer needs to double check?

N/A

Why was this MR needed?

It's mainly intended to improve Elasticsearch indexing performance (through multi_json gem), but other gems could benefit from it too, like grape. For ES, indexing Linux kernel commits went from 766 to 519 seconds after using this gem.

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

N/A