Fix Ruby version compatibility by always using buildpack Ruby

[ramonskie]

Summary

• Fixes staging failure "unknown keywords: :permitted_classes, :aliases" reported in Migrate to Switchblade integration tests and modernize Ruby toolchain #1128 after Ruby 3.4.7 migration (Migration to ruby 3.4.7 #1127)
• Ensures buildpack always uses its own Ruby 3.2.8 (with Psych 5.0.1+) instead of falling back to cflinuxfs4's older system Ruby

Details

After merging PR #1127, users reported staging failures in Cloud Foundry with the error:

ERR unknown keywords: :permitted_classes, :aliases
ERR Failed to compile droplet: Failed to run finalize script: exit status 1

Root Cause: The buildpack's bin/run script checked if system Ruby exists before installing its own Ruby. When Cloud Foundry's cflinuxfs4 stack provided an older system Ruby (with Psych < 5.0), the buildpack used it instead of downloading Ruby 3.2.8. The older Psych version doesn't support the permitted_classes: and aliases: keyword arguments added in the migration.

Fix: Modified bin/run to always install and use the buildpack's own Ruby 3.2.8, removing the conditional check for system Ruby. This ensures consistent behavior across all environments.

Changes

• bin/run: Always install buildpack Ruby instead of checking for system Ruby
• config/ruby.yml: Added documentation comment about Ruby 3.2.8+/Psych 5.0.1+ requirement

[wjonkheijm]

I tested a push with this branch in our cflinuxfs4-compat environment and it worked for me. push/restage fine.