Update json requirement from 2.7.6 to 2.8.1

Updates the requirements on [json](https://github.com/ruby/json) to permit the latest version.
- [Release notes](https://github.com/ruby/json/releases)
- [Changelog](https://github.com/ruby/json/blob/master/CHANGES.md)
- [Commits](ruby/json@v2.7.6...v2.8.1)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Gemfile

@@ -3,7 +3,7 @@ source 'https://rubygems.org'
 gem 'rake', '13.2.1'
 gem 'asciidoctor', '2.0.22'
 
-gem 'json', '2.7.6'
+gem 'json', '2.8.1'
 gem 'awesome_print', '1.9.2'
 
 gem 'asciidoctor-fb2', '0.7.0'