Use casecmp() instead of casecmp?() for now (#4832)

* Use casecmp() instead of casecmp?() for now

casecmp?() is only available in ruby 2.4.0.  Users running earlier ruby versions
would see errors, e.g., running
RAILS_ENV=production rails mastodon:maintenance:remove_deprecated_preview_cards.

* Correctly check whether casecmp() returns 0