Default new Ruby scrapers to heroku-24 instead of heroku-18

[Br3nda]

What does this do?

Changes the platform file in the Ruby scraper template (used when a new scraper is created via morph.io's "create a scraper" flow) from heroku-18 to heroku-24.

Why was this needed?

heroku-18's buildstep image hasn't been rebuilt since 2022 - CI build/push for it (and cedar-14) was disabled on 2025-05-21 (see openaustralia/buildstep's workflow files, "disabling the older builds so the existing working images don't get disturbed"). heroku-24 is the only stack still actively rebuilt.

Tested a real Ruby scraper end-to-end against the current openaustralia/buildstep:heroku-24 image: clean bundle install, no buildpack issues.

Scope - deliberately narrow

• Only affects brand-new Ruby scrapers created from this point forward. default_files/ruby/template/platform is read once, at creation time, by CreateScraperWorker - existing scrapers (with or without their own platform file) are completely unaffected.
• DEFAULT_PLATFORM (cedar-14, used when a scraper has no platform file at all) is untouched - so old scrapers relying on that implicit default keep behaving exactly as they do today.
• Ruby only. Node/PHP/Perl are known or suspected broken on heroku-24 (see the existing # FIXME: get php and perl examples / buildstep working for heroku-24 in Language, and openaustralia/morph#1443/#1444/#1448) and haven't been individually re-verified - happy to do the same test-first pass on each if useful.
• default_files/python/template/platform was already heroku-24 before this PR.

Testing done

herokuish buildpack build against a real scraper (unmodified Gemfile pinning ruby "3.2.2") run directly against openaustralia/buildstep:heroku-24, pulled fresh:

-----> Ruby app detected
-----> Using Ruby version: ruby-3.2.2
Bundle complete! 3 Gemfile dependencies, 21 gems now installed.

Opening as a draft while I sanity-check whether there's anything else worth doing alongside this (e.g. whether update_docker_images! needs to run for the change to actually take effect on morph.io's build hosts).

[benrfairless]

This can't be done while we are on the old server. It will need to wait until we are on the new server with newer docker.