Warn on default ruby

Should tell you to add a `ruby ''` declaration and run `bundle update --ruby`.

```
remote: # Heroku Ruby Buildpack
remote:
remote: - Metrics agent
remote: - Skipping install (`barnes` gem not found)
remote: - Ruby version `3.1.3` from `default`
remote: - Installing .... (1.447s)
remote: - Bundler version `2.5.17` from `Gemfile.lock`
remote: - Running `gem install bundler --version 2.5.17` ... (0.656s)
remote: - Bundle install
remote: - 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`
remote:
remote: Fetching gem metadata from https://rubygems.org/.........
remote: zeitwerk-2.7.1 requires ruby version >= 3.2, which is incompatible with the
remote: current version, 3.1.3
remote:
remote: - Done (3.275s)
remote: - Debug info
remote: - Command failed `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`
remote: exit status: 5
remote: stdout:
remote: stderr:
remote:
remote: ! Error installing your applications's dependencies
remote: !
remote: ! Could not install gems to the system via bundler. Gems are dependencies
remote: ! your application listed in the `Gemfile` and resolved in the `Gemfile.lock`.
remote: !
remote: ! Ensure you can run the following command locally with no errors before attempting another build:
remote: !
remote: ! `BUNDLE_BIN="/layers/heroku_ruby/gems/bin" BUNDLE_CLEAN="1" BUNDLE_DEPLOYMENT="1" BUNDLE_GEMFILE="./Gemfile" BUNDLE_PATH="/layers/heroku_ruby/gems" BUNDLE_WITHOUT="development:test" bundle install`
remote: !
remote: !
remote: !
remote: ! If you believe that your application is correct, ensure all files are tracked in Git and
remote: ! that you're pushing the correct branch:
remote: ! https://devcenter.heroku.com/articles/git#deploy-from-a-branch-besides-main
remote: !
remote: ! Use the information above to debug further.
remote:
remote: ERROR: failed to build: exit status 1
remote: Verifying deploy...
remote:
remote: ! Push rejected to .
remote:
```