build(deps): avoid loading json 2.7.0

[Uaitt]

Avoid loading release 2.7.0 of the json gem as it contains a backward-compatibility issue.

(Basically, on 2.7.0 you can't do JSON.dump(a:1, b:2, c:3) but you need to do JSON.dump({ a:1, b:2, c:3 })

Refrences:

• Issue => JSON.dump does not work in version 2.7.0 when the argument is an implicit Hash ruby/json#553
• PR => JSON.dump: handle unenclosed hashes regression ruby/json#554 and Overload kwargs in JSON.dump ruby/json#556

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[Uaitt]

Not sure if I should add a Changelog entry for this 🤔

[koic]

Tests for RuboCop pass with the matrix of JSON 2.7.0 and Ruby 2.7, and it works. This means that instead of avoiding the version of JSON on the RuboCop side, it seems best to handle it on applications and client side. Thank you.