Bump puma from 4.2.1 to 4.3.1 in /website (#100)

Bumps [puma](https://github.com/puma/puma) from 4.2.1 to 4.3.1.
- [Release notes](https://github.com/puma/puma/releases)
- [Changelog](https://github.com/puma/puma/blob/master/History.md)
- [Commits](puma/puma@v4.2.1...v4.3.1)

This is just for the demo website.

website/Gemfile.lock

@@ -8,7 +8,7 @@ GEM
   specs:
     mustermann (1.0.3)
     nio4r (2.5.2)
-    puma (4.2.1)
+    puma (4.3.1)
       nio4r (~> 2.0)
     rack (2.0.7)
     rack-protection (2.0.7)