Bump rack from 2.0.8 to 2.2.3 in /example-buildpack

Bumps [rack](https://github.com/rack/rack) from 2.0.8 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@2.0.8...2.2.3)

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

example-buildpack/Gemfile.lock

@@ -2,7 +2,7 @@ GEM
   remote: https://rubygems.org/
   specs:
     mustermann (1.0.3)
-    rack (2.0.8)
+    rack (2.2.3)
     rack-protection (2.0.7)
       rack
     sinatra (2.0.7)