[Security] Bump rack from 2.0.7 to 2.0.8 in /backend

Bumps [rack](https://github.com/rack/rack) from 2.0.7 to 2.0.8. **This update includes a security fix.**
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](rack/rack@2.0.7...2.0.8)

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

Author: dependabot-preview[bot]

backend/Gemfile.lock

@@ -106,7 +106,7 @@ GEM
     public_suffix (4.0.1)
     puma (4.3.1)
       nio4r (~> 2.0)
-    rack (2.0.7)
+    rack (2.0.8)
     rack-cors (1.1.0)
       rack (>= 2.0.0)
     rack-test (1.1.0)