Heroku fix (#19807)

Currently building on Heroku fails with a package error for libidn11. Since Mastodon uses idn-ruby 0.1.2 we should upgrade to libidn12

Aptfile

 ffmpeg
 libicu[0-9][0-9]
 libicu-dev
-libidn11
-libidn11-dev
+libidn12
+libidn-dev
 libpq-dev
 libxdamage1
 libxfixes3