Update fixtures for Ruby 3.2+ compatibility

- Upgrade Rails 6.1 -> Rails 7.2 in test fixtures
  - Rename rails6_sprockets -> rails72_sprockets
  - Rename multibuildpack/rails6 -> multibuildpack/rails72
  - Rails 6.1 is EOL (June 2024) and has Ruby 3.2 compatibility issues

- Add rackup gem to all Rack-based applications
  - Required since rackup was extracted from Ruby stdlib in Ruby 3.0+
  - Added to Sinatra, custom Gemfile, and multibuildpack fixtures
  - Updated vendor/cache for fixtures that use cached gems

- Update JRuby fixture for Ruby 3.2 compatibility
  - Upgrade gem dependencies (Sinatra 3.0 -> 4.2, Rack 2.2 -> 3.2)
  - Add universal-java-1.8 platform to Gemfile.lock
  - Regenerate lockfile and vendor/cache with JRuby 9.4.8.0

- Regenerate all Gemfile.lock files with Ruby 3.2.9

All integration tests passing (15/15)

Author: ramonskie

fixtures/default/custom_gemfile/Gemfile-APP

@@ -1,5 +1,6 @@
 source 'https://rubygems.org'
 ruby "~>3.2.0"
 gem "cf-app-utils"
+gem 'rackup'
 gem "webrick"
 gem "sinatra"

fixtures/default/custom_gemfile/Gemfile-APP.lock

@@ -1,28 +1,43 @@
 GEM
   remote: https://rubygems.org/
   specs:
+    base64 (0.3.0)
     cf-app-utils (0.6)
-    mustermann (3.0.0)
+    logger (1.7.0)
+    mustermann (3.0.4)
       ruby2_keywords (~> 0.0.1)
-    rack (2.2.4)
-    rack-protection (3.0.4)
-      rack
+    rack (3.2.4)
+    rack-protection (4.2.1)
+      base64 (>= 0.1.0)
+      logger (>= 1.6.0)
+      rack (>= 3.0.0, < 4)
+    rack-session (2.1.1)
+      base64 (>= 0.1.0)
+      rack (>= 3.0.0)
+    rackup (2.3.1)
+      rack (>= 3)
     ruby2_keywords (0.0.5)
-    sinatra (3.0.4)
+    sinatra (4.2.1)
+      logger (>= 1.6.0)
       mustermann (~> 3.0)
-      rack (~> 2.2, >= 2.2.4)
-      rack-protection (= 3.0.4)
+      rack (>= 3.0.0, < 4)
+      rack-protection (= 4.2.1)
+      rack-session (>= 2.0.0, < 3)
       tilt (~> 2.0)
-    tilt (2.0.11)
-    webrick (1.7.0)
+    tilt (2.7.0)
+    webrick (1.9.2)
 
 PLATFORMS
-  ruby
+  x86_64-linux
 
 DEPENDENCIES
   cf-app-utils
+  rackup
   sinatra
   webrick
 
 RUBY VERSION
-   ruby 3.1.3p185
+   ruby 3.2.9p265
+
+BUNDLED WITH
+   2.4.19