Upgrade Puma 5 → 6 for Rack 3 compatibility

Puma 5 is not compatible with Rack 3 which was pulled in by the
Rails 7.2 upgrade.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mikeland73

examples/stacks/rails/blog/Gemfile

@@ -13,7 +13,7 @@ gem "sprockets-rails"
 gem "sqlite3", "~> 1.4"
 
 # Use the Puma web server [https://github.com/puma/puma]
-gem "puma", "~> 5.6"
+gem "puma", "~> 6.0"
 
 # Use JavaScript with ESM import maps [https://github.com/rails/importmap-rails]
 gem "importmap-rails"

examples/stacks/rails/blog/Gemfile.lock

@@ -164,7 +164,7 @@ GEM
       date
       stringio
     public_suffix (7.0.5)
-    puma (5.6.9)
+    puma (6.6.1)
       nio4r (~> 2.0)
     racc (1.8.1)
     rack (3.2.5)
@@ -274,7 +274,7 @@ DEPENDENCIES
   debug
   importmap-rails
   jbuilder
-  puma (~> 5.6)
+  puma (~> 6.0)
   rails (~> 7.2.3.1)
   selenium-webdriver
   sprockets-rails