Bump json from 2.2.0 to 2.3.1 in /ios

Bumps [json](https://github.com/flori/json) from 2.2.0 to 2.3.1.
- [Release notes](https://github.com/flori/json/releases)
- [Changelog](https://github.com/flori/json/blob/master/CHANGES.md)
- [Commits](ruby/json@v2.2.0...v2.3.1)

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

ios/Gemfile.lock

@@ -93,7 +93,7 @@ GEM
     http-cookie (1.0.3)
       domain_name (~> 0.5)
     httpclient (2.8.3)
-    json (2.2.0)
+    json (2.3.1)
     jwt (2.1.0)
     memoist (0.16.0)
     mime-types (3.2.2)