Bump rack from 3.1.8 to 3.1.12 in /examples/sinatra/app/src (#582)

Bumps [rack](https://github.com/rack/rack) from 3.1.8 to 3.1.12.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@v3.1.8...v3.1.12)

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

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examples/sinatra/app/src/Gemfile.lock

@@ -5,7 +5,7 @@ GEM
     logger (1.6.1)
     mustermann (3.0.3)
       ruby2_keywords (~> 0.0.1)
-    rack (3.1.8)
+    rack (3.1.12)
     rack-protection (4.1.0)
       base64 (>= 0.1.0)
       logger (>= 1.6.0)