Bump json from 2.15.0 to 2.15.2.1 in /testapp

Bumps [json](https://github.com/ruby/json) from 2.15.0 to 2.15.2.1.
- [Release notes](https://github.com/ruby/json/releases)
- [Changelog](https://github.com/ruby/json/blob/master/CHANGES.md)
- [Commits](ruby/json@v2.15.0...v2.15.2.1)

---
updated-dependencies:
- dependency-name: json
  dependency-version: 2.15.2.1
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

testapp/Gemfile.lock

@@ -79,7 +79,7 @@ GEM
       mutex_m
     i18n (1.14.7)
       concurrent-ruby (~> 1.0)
-    json (2.15.0)
+    json (2.15.2.1)
     logger (1.7.0)
     minitest (5.25.5)
     molinillo (0.8.0)