Bump json from 2.15.0 to 2.15.2.1 in /testapp (#366)

Author: dependabot[bot]

testapp/Gemfile.lock

@@ -79,7 +79,7 @@ GEM
       mutex_m
     i18n (1.14.8)
       concurrent-ruby (~> 1.0)
-    json (2.15.0)
+    json (2.15.2.1)
     logger (1.7.0)
     minitest (5.27.0)
     molinillo (0.8.0)