[Security] Bump puma from 4.3.0 to 4.3.1 in /backend

Bumps [puma](https://github.com/puma/puma) from 4.3.0 to 4.3.1. **This update includes a security fix.**
- [Release notes](https://github.com/puma/puma/releases)
- [Changelog](https://github.com/puma/puma/blob/master/History.md)
- [Commits](puma/puma@v4.3.0...v4.3.1)

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

Author: dependabot-preview[bot]

backend/Gemfile.lock

@@ -105,7 +105,7 @@ GEM
       mini_portile2 (~> 2.4.0)
     pg (1.1.4)
     public_suffix (4.0.1)
-    puma (4.3.0)
+    puma (4.3.1)
       nio4r (~> 2.0)
     rack (2.0.7)
     rack-cors (1.0.6)