Update json requirement from 2.5.1 to 2.6.1 (progit#262)

Updates the requirements on [json](https://github.com/flori/json) to permit the latest version.
- [Release notes](https://github.com/flori/json/releases)
- [Changelog](https://github.com/flori/json/blob/master/CHANGES.md)
- [Commits](ruby/json@v2.5.1...v2.6.1)

---
updated-dependencies:
- dependency-name: json
  dependency-type: direct:production
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Gemfile

@@ -3,7 +3,7 @@ source 'https://rubygems.org'
 gem 'rake', '13.0.6'
 gem 'asciidoctor', '2.0.16'
 
-gem 'json', '2.5.1'
+gem 'json', '2.6.1'
 gem 'awesome_print', '1.9.2'
 
 gem 'asciidoctor-epub3', '1.5.1'