Bump rack from 3.1.18 to 3.1.20 in /demo

Bumps [rack](https://github.com/rack/rack) from 3.1.18 to 3.1.20.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@v3.1.18...v3.1.20)

---
updated-dependencies:
- dependency-name: rack
  dependency-version: 3.1.20
  dependency-type: indirect
...

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

Author: dependabot[bot]

demo/Gemfile.lock

@@ -34,7 +34,7 @@ GEM
     puma (6.5.0)
       nio4r (~> 2.0)
     racc (1.8.1)
-    rack (3.1.18)
+    rack (3.1.20)
     rack-test (2.2.0)
       rack (>= 1.3)
     rackup (2.2.1)