Bump rack from 2.2.21 to 2.2.22 in /backend

Bumps [rack](https://github.com/rack/rack) from 2.2.21 to 2.2.22.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@v2.2.21...v2.2.22)

---
updated-dependencies:
- dependency-name: rack
  dependency-version: 2.2.22
  dependency-type: indirect
...

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

Author: dependabot[bot]

backend/Gemfile.lock

@@ -343,7 +343,7 @@ GEM
       pusher-signature (~> 0.1.8)
     pusher-signature (0.1.8)
     racc (1.8.1)
-    rack (2.2.21)
+    rack (2.2.22)
     rack-cors (2.0.1)
       rack (>= 2.0.0)
     rack-session (1.0.2)