Bump rack from 2.2.6.4 to 2.2.8.1 in /examples/sinatra

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]>

examples/sinatra/Gemfile.lock

@@ -101,7 +101,7 @@ GEM
     public_suffix (4.0.6)
     puma (5.6.7)
       nio4r (~> 2.0)
-    rack (2.2.6.4)
+    rack (2.2.8.1)
     rack-protection (2.2.3)
       rack
     rake (13.0.6)