Bundler default version now 2.3.x

Bundler 1.x is not able to run with Ruby 3.3 in some circumstances:

```
       remote:  !     There was an error parsing your Gemfile, we cannot continue        
       remote:  !     /tmp/d20250127-138-l8vq2r/bundler-1.17.3/gems/bundler-1.17.3/lib/bundler/shared_helpers.rb:29:in `root': undefined method `untaint' for an instance of Pathname (NoMethodError)        
       remote:  !             
       remote:  !     Pathname.new(gemfile).untaint.expand_path.parent  
```

This commit updates the default bundler version to 2.3.x.

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## [Unreleased]
 
 - Default Ruby version is now 3.3.7 (https://github.com/heroku/heroku-buildpack-ruby/pull/1534)
+- Default bundler version (when no version present in the `Gemfile.lock`) is now bundler `2.3.x` which is currently `2.3.25` (https://github.com/heroku/heroku-buildpack-ruby/pull/1534)
 
 ## [v288] - 2025-01-06
 

changelogs/unreleased/bundler_default.md

@@ -0,0 +1,13 @@
+## Ruby applications with no specified bundler versions now receive Bundler 2.x
+
+Previously applications with no `BUNDLED WITH` in their `Gemfile.lock` would receive bundler `1.x`. They will now receive the new [default bundler version](https://devcenter.heroku.com/articles/ruby-support-reference#default-bundler-version) `2.3.x`.
+
+It is strongly recommended that you have both a `RUBY VERSION` and `BUNDLED WITH` version listed in your `Gemfile.lock`. If you do not have those values, you can generate them and commit them to git:
+
+```
+$ bundle update --ruby
+$ git add Gemfile.lock
+$ git commit -m "Update Gemfile.lock"
+```
+
+Applications without these values specified in the `Gemfile.lock` may break unexpectedly when the defaults change.

lib/language_pack/helpers/bundler_wrapper.rb

@@ -42,6 +42,10 @@ class LanguagePack::Helpers::BundlerWrapper
   BLESSED_BUNDLER_VERSIONS["2.4"] = "2.4.22"
   BLESSED_BUNDLER_VERSIONS["2.5"] = "2.5.23"
   BLESSED_BUNDLER_VERSIONS["2.6"] = "2.6.2"
+
+  DEFAULT_VERSION = BLESSED_BUNDLER_VERSIONS["2.3"]
+
+  # Convert arbitrary `<Major>.<Minor>.x` versions
   BLESSED_BUNDLER_VERSIONS.default_proc = Proc.new do |hash, key|
     if Gem::Version.new(key).segments.first == 1
       hash["1"]
@@ -65,7 +69,7 @@ def self.detect_bundler_version(contents: )
       minor = version_match[:minor]
       BLESSED_BUNDLER_VERSIONS["#{major}.#{minor}"]
     else
-      BLESSED_BUNDLER_VERSIONS["1"]
+      DEFAULT_VERSION
     end
   end