build(deps): bump json from 2.7.6 to 2.8.1 in /assets/service_broker

Bumps [json](https://github.com/ruby/json) from 2.7.6 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@v2.7.6...v2.8.1)

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

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

assets/service_broker/Gemfile.lock

@@ -3,7 +3,7 @@ GEM
   specs:
     base64 (0.2.0)
     diff-lcs (1.5.1)
-    json (2.7.6)
+    json (2.8.1)
     mustermann (3.0.0)
       ruby2_keywords (~> 0.0.1)
     rack (3.1.8)