Bump json from 2.7.5 to 2.8.1

Bumps [json](https://github.com/ruby/json) from 2.7.5 to 2.8.1.
- [Release notes](https://github.com/ruby/json/releases)
- [Changelog](https://github.com/ruby/json/blob/master/CHANGES.md)
- [Commits](ruby/json@v2.7.5...v2.8.1)

---
updated-dependencies:
- dependency-name: json
  dependency-type: indirect
  update-type: version-update:semver-minor
...

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

Gemfile.lock

@@ -127,7 +127,7 @@ GEM
     i18n (1.6.0)
       concurrent-ruby (~> 1.0)
     io-event (1.7.3)
-    json (2.7.5)
+    json (2.8.1)
     kramdown (2.4.0)
       rexml
     listen (3.9.0)