Bump rackup from 2.2.1 to 2.3.1 in /builder in the rails group (#2868)

Bumps the rails group in /builder with 1 update: [rackup](https://github.com/rack/rackup).


Updates `rackup` from 2.2.1 to 2.3.1
- [Release notes](https://github.com/rack/rackup/releases)
- [Changelog](https://github.com/rack/rackup/blob/main/releases.md)
- [Commits](rack/rackup@v2.2.1...v2.3.1)

---
updated-dependencies:
- dependency-name: rackup
  dependency-version: 2.3.1
  dependency-type: indirect
  update-type: version-update:semver-minor
  dependency-group: rails
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

builder/Gemfile.lock

@@ -214,7 +214,7 @@ GEM
       rack (>= 3.0.0)
     rack-test (2.2.0)
       rack (>= 1.3)
-    rackup (2.2.1)
+    rackup (2.3.1)
       rack (>= 3)
     rails (8.1.1)
       actioncable (= 8.1.1)