Bump rack from 2.0.3 to 2.2.3 in /example_app

Bumps [rack](https://github.com/rack/rack) from 2.0.3 to 2.2.3.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](rack/rack@2.0.3...2.2.3)

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

Author: dependabot[bot]

example_app/Gemfile.lock

@@ -39,7 +39,7 @@ GEM
       coderay (~> 1.1.0)
       method_source (~> 0.9.0)
     public_suffix (3.0.0)
-    rack (2.0.3)
+    rack (2.2.3)
     rack-flash3 (1.0.5)
       rack
     rack-protection (2.0.0)