chore(deps): bump rack in /buildpacks/buildpack-multi/tests/multi

Bumps [rack](https://github.com/rack/rack) from 3.1.4 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@v3.1.4...v3.1.5)

---
updated-dependencies:
- dependency-name: rack
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

buildpacks/buildpack-multi/tests/multi/Gemfile.lock

@@ -4,7 +4,7 @@ GEM
     base64 (0.2.0)
     mustermann (3.0.0)
       ruby2_keywords (~> 0.0.1)
-    rack (3.1.4)
+    rack (3.1.5)
     rack-protection (4.0.0)
       base64 (>= 0.1.0)
       rack (>= 3.0.0, < 4)