Bump rack from 3.1.17 to 3.1.18 in /demo (primer#3730)

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Hector Garcia <hectahertz@github.com>
Co-authored-by: Jon Rohan <rohan@github.com>

Author: 3 people

demo/Gemfile.lock

@@ -263,7 +263,7 @@ GEM
     puma (6.6.1)
       nio4r (~> 2.0)
     racc (1.8.1)
-    rack (3.1.17)
+    rack (3.1.18)
     rack-cors (3.0.0)
       logger
       rack (>= 3.0.14)