Update bundler version warning (heroku#1697)

Previously the output would look like this:

```
�[1m�[33m###### WARNING:�[0m

       Your app was upgraded to bundler 2.3.7.
       Previously you had a successful deploy with bundler 2.3.25.
       
       If you see problems related to the bundler version please refer to:
       https://devcenter.heroku.com/articles/bundler-version#known-upgrade-issues
       

-----> Using Ruby version: ruby-3.1.2
-----> Installing bundler 2.3.7
-----> Installing dependencies using bundler 2.3.7
       Running: BUNDLE_WITHOUT='development:test' BUNDLE_PATH=vendor/bundle BUNDLE_BIN=vendor/bundle/bin BUNDLE_DEPLOYMENT=1 bundle install -j4
       [1121, #<Thread:0x00007f1ff87f7c58 run>, #<NameError: uninitialized constant Gem::Source
       
             (defined?(@source) && @source) || Gem::Source::Installed.new
                                                  ^^^^^^^^
       Did you mean?  Gem::SourceList>, 
```

Now it directs them on how to fix the problem directly.

Author: schneems

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## [Unreleased]
 
+- Update bundler version warning output (https://github.com/heroku/heroku-buildpack-ruby/pull/1697)
 
 ## [v341] - 2026-01-09
 

lib/language_pack/ruby.rb

@@ -172,12 +172,22 @@ def self.warn_bundler_upgrade(metadata: , bundler_version: )
 
     if old_bundler_version && old_bundler_version != bundler_version
       warn(<<~WARNING, inline: true)
-        Your app was upgraded to bundler #{ bundler_version }.
-        Previously you had a successful deploy with bundler #{ old_bundler_version }.
+        Your bundler version changed from `#{old_bundler_version}` to `#{bundler_version}`.
 
-        If you see problems related to the bundler version please refer to:
-        https://devcenter.heroku.com/articles/bundler-version#known-upgrade-issues
+        If you see problems related to the new bundler version, you can revert to the previous version
+        by updating the `BUNDLED WITH` value in your `Gemfile.lock`. For example:
 
+        ```
+        BUNDLED WITH
+           #{old_bundler_version}
+        ```
+
+        Commit the results to git before redeploying:
+
+        ```
+        $ git add Gemfile.lock
+        $ git commit -m "Revert to previous bundler version"
+        ```
       WARNING
     end
   end