Bump rack from 2.2.9 to 2.2.20 in /backend

Bumps [rack](https://github.com/rack/rack) from 2.2.9 to 2.2.20.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@v2.2.9...v2.2.20)

---
updated-dependencies:
- dependency-name: rack
  dependency-version: 2.2.20
  dependency-type: indirect
...

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

Author: dependabot[bot]

backend/Gemfile.lock

@@ -331,7 +331,7 @@ GEM
       pusher-signature (~> 0.1.8)
     pusher-signature (0.1.8)
     racc (1.8.0)
-    rack (2.2.9)
+    rack (2.2.20)
     rack-cors (2.0.1)
       rack (>= 2.0.0)
     rack-session (1.0.2)