Bump puma from 3.12.1 to 3.12.6 in /app

Bumps [puma](https://github.com/puma/puma) from 3.12.1 to 3.12.6.
- [Release notes](https://github.com/puma/puma/releases)
- [Changelog](https://github.com/puma/puma/blob/master/History.md)
- [Commits](puma/puma@v3.12.1...v3.12.6)

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

Author: dependabot[bot]

app/Gemfile.lock

@@ -32,7 +32,7 @@ GEM
     mustermann (1.0.3)
     mysql2 (0.5.2)
     pg (0.18.4)
-    puma (3.12.1)
+    puma (3.12.6)
     rack (2.2.3)
     rack-protection (2.0.5)
       rack