test: upgrade Ruby's bundler to make it work with modern Rubies (#4132)

bundler 2.0.2 is too old for Ruby 3.2.3, due to its untaint usage.

rubygems/bundler#7385
https://github.com/actions/runner-images/blob/ubuntu24/20241215.1/images/ubuntu/Ubuntu2404-Readme.md#language-and-runtime

test/preflight/fixtures/example-buildpack/Gemfile.lock

@@ -26,4 +26,4 @@ DEPENDENCIES
   sinatra
 
 BUNDLED WITH
-   2.0.2
+   2.6.1