Make govuk_app_config use Puma 5

The capybara gem is currently not compatible with Puma 6 and thus this
gem update can't go ahead.

Capybara already has a fix for this which hasn't been released [1]

[1]: teamcapybara/capybara#2530

Author: chao-xian

Gemfile.lock

@@ -225,7 +225,7 @@ GEM
       coderay (~> 1.1)
       method_source (~> 1.0)
     public_suffix (5.0.0)
-    puma (6.0.0)
+    puma (5.6.5)
       nio4r (~> 2.0)
     racc (1.6.0)
     rack (2.2.4)