Bump rack from 2.2.6.4 to 2.2.8.1 in /example-buildpack (#3938)

Bumps [rack](https://github.com/rack/rack) from 2.2.6.4 to 2.2.8.1.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@v2.2.6.4...v2.2.8.1)

---
updated-dependencies:
- dependency-name: rack
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

example-buildpack/Gemfile.lock

@@ -3,7 +3,7 @@ GEM
   specs:
     mustermann (2.0.2)
       ruby2_keywords (~> 0.0.1)
-    rack (2.2.6.4)
+    rack (2.2.8.1)
     rack-protection (2.2.3)
       rack
     ruby2_keywords (0.0.5)