Bump rack from 3.1.14 to 3.1.18 in /demo

Bumps [rack](https://github.com/rack/rack) from 3.1.14 to 3.1.18.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@v3.1.14...v3.1.18)

---
updated-dependencies:
- dependency-name: rack
  dependency-version: 3.1.18
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

demo/Gemfile.lock

@@ -21,7 +21,7 @@ GEM
     puma (6.6.0)
       nio4r (~> 2.0)
     racc (1.8.1)
-    rack (3.1.14)
+    rack (3.1.18)
     rack-test (2.2.0)
       rack (>= 1.3)
     rackup (2.2.1)