Update jruby version for integration test

Older versions of JRuby do not play well with newer versions of bundler:

```
  - Running `BUNDLE_BIN="/layers/heroku_ruby/gems/bin" BUNDLE_CLEAN="1" BUNDLE_DEPLOYMENT="1" BUNDLE_GEMFILE="/workspace/Gemfile" BUNDLE_PATH="/layers/heroku_ruby/gems" BUNDLE_WITHOUT="development:test" bundle install`

      Picked up JAVA_TOOL_OPTIONS: -Dfile.encoding=UTF-8
      NoMethodError: undefined method `request' for nil:NilClass
      Did you mean?  require
        conflicting_dependencies at /layers/heroku_ruby/ruby/lib/ruby/stdlib/rubygems/resolver/conflict.rb:47
        conflicting_dependencies at /layers/heroku_ruby/ruby/lib/ruby/stdlib/rubygems/exceptions.rb:32
                      initialize at /layers/heroku_ruby/ruby/lib/ruby/stdlib/rubygems/exceptions.rb:26
                             new at org/jruby/RubyClass.java:909
                       exception at org/jruby/RubyException.java:129
                         resolve at /layers/heroku_ruby/ruby/lib/ruby/stdlib/rubygems/resolver.rb:193
                         resolve at /layers/heroku_ruby/ruby/lib/ruby/stdlib/rubygems/request_set.rb:411
                 resolve_current at /layers/heroku_ruby/ruby/lib/ruby/stdlib/rubygems/request_set.rb:423
                  finish_resolve at /layers/heroku_ruby/ruby/lib/ruby/stdlib/rubygems.rb:240
               activate_bin_path at /layers/heroku_ruby/ruby/lib/ruby/stdlib/rubygems.rb:300
                     synchronize at org/jruby/ext/thread/Mutex.java:171
               activate_bin_path at /layers/heroku_ruby/ruby/lib/ruby/stdlib/rubygems.rb:298
                          <main> at /layers/heroku_ruby/bundler/bin/bundle:23
```

buildpacks/ruby/tests/integration_test.rs

@@ -148,7 +148,7 @@ fn test_jruby_app() {
         r#"
         source "https://rubygems.org"
 
-        ruby '2.6.8', engine: 'jruby', engine_version: '9.3.6.0'
+        ruby '3.1.4', engine: 'jruby', engine_version: '9.4.8.0'
     "#,
     )
     .unwrap();
@@ -162,7 +162,7 @@ GEM
 PLATFORMS
   java
 RUBY VERSION
-   ruby 2.6.8p001 (jruby 9.3.6.0)
+   ruby 3.1.4p001 (jruby 9.4.8.0)
 DEPENDENCIES
 ",
     )
@@ -183,7 +183,7 @@ DEPENDENCIES
                 context.pack_stdout,
                 r#"`BUNDLE_BIN="/layers/heroku_ruby/gems/bin" BUNDLE_CLEAN="1" BUNDLE_DEPLOYMENT="1" BUNDLE_GEMFILE="/workspace/Gemfile" BUNDLE_PATH="/layers/heroku_ruby/gems" BUNDLE_WITHOUT="development:test" bundle install`"#
             );
-            assert_contains!(context.pack_stdout, "Ruby version `2.6.8-jruby-9.3.6.0` from `Gemfile.lock`");
+            assert_contains!(context.pack_stdout, "Ruby version `3.1.4-jruby-9.4.8.0` from `Gemfile.lock`");
             });
 }