Bump rack from 3.1.3 to 3.1.5 in /test/app

Bumps [rack](https://github.com/rack/rack) from 3.1.3 to 3.1.5.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@v3.1.3...v3.1.5)

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

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

test/app/Gemfile.lock

@@ -7,7 +7,7 @@ GEM
     nio4r (2.7.3)
     puma (6.4.2)
       nio4r (~> 2.0)
-    rack (3.1.3)
+    rack (3.1.5)
     rack-protection (4.0.0)
       base64 (>= 0.1.0)
       rack (>= 3.0.0, < 4)