Bump rack from 2.0.7 to 2.2.3 in /app

Bumps [rack](https://github.com/rack/rack) from 2.0.7 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@2.0.7...2.2.3)

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

Author: dependabot[bot]

app/Gemfile.lock

@@ -33,7 +33,7 @@ GEM
     mysql2 (0.5.2)
     pg (0.18.4)
     puma (3.12.1)
-    rack (2.0.7)
+    rack (2.2.3)
     rack-protection (2.0.5)
       rack
     sequel (5.19.0)